As it turned out, he was right, and he wasnt a Murphy.
If not spray foam, then the poured foam, anchor the ball in a box and pour the foam around it perhaps…
Epoxy and foam here, non affiliated link of course….
There are a lot of ideas here. Some I might try, some I would not. Cost is an issue. Newspaper, Elmer's glue and a $10 ball are not too bad of a price, but many of the other ways are really out of the budget. I got answers on plastic spheres. Twice what I had imagines. Try $700-800 for a 2 piece 36" sphere or $400 for a 1 piece.
Foam in aerosol cans could run me about $50-70 for a thin layer. I did find an interesting video for an adapter, to make the spray can into a wide area dispenser.
He sells them on ebay. http://www.ebay.com/itm/NEW-PRODUCT-Vertical-Foam-Nozzle-DIY-spray-foam-substitute-smaller-jobs-/151706012563?
It might be a possibility.
The larger containers of 2 part foam, such as gallon containers, are also very expensive. More than I would consider.
I think the metal domes from china would be very expensive to ship to the US. The ones made of stainless that I find in the US, originally made in China, are very high.
The resin and fiberglass mesh is an idea I seriously considered, but the fumes are a bit much. I wouldn't be buying a respirator and from my early years of painting cars with no mask, I can't do it with just a dust mask, so that stuff is eliminated, but it is a valid idea and should make a nice sphere.
Paper mache, I believe, was too optimistic and naive for a large sphere. For a small one, it would probably be ok. It was not a great idea, just a cheaper idea.
As far as being truly round and smooth, well, look at real spheres and find the answer to that one. I have a limit where ok and not ok are and my limit is probably more strict than many, but it is what it is.
As always, the posts and conversations are welcome and helpful. Whether I do anything or not, the thoughts put into the thread help all of us and give me more options to think about.
I really like the idea of the poured foam. It's just the cost. Say I went to HD or Lowe's and had them cut plywood for me. Make two squares say, 40" x 40" with one open side on each. Cut a 6" or 8" plastic planter and use it as the large portal. Set it in the bottom box, set the sphere on it and make it so the center of the sphere is just above the top of the box, by a 1/16" or so. Cut a 48" x 48" piece of sheet metal so it fits the center of the sphere when it is on top of the bottom box. Set it on top of the bottom box, so the sphere is centered and set the top box on. Drill one hole in the center of the top, so a 1/2" rod can come down and be used to hold the sphere in, so it does not try to float up. Drill possibly two or four pour holes, much like a casting and pour in the mix. Probably you would also need to strap the two boxes together, so they did not separate. It probably would work, but the cost is now in the hundreds. Love the idea, but not the price.
What about finding a local spray foam insulator and making a connection with them, I know they have to purge their lines at the end of a job, may be enough for what you want. May cost you a flashlight in trade or something along those lines. Your labor is more than the foam cost, whether you value your time at that rate or not, you can get more money, but your time is finite…. Keep looking, something acceptable will turn up…
oh wow the price for those plastic sphere’s are outrageous!
Maybe you should put and AD in craigs list looking for an old disco ball. :bigsmile:
This might be an option with a little tweaking?
http://www.artistic-garden.com/garden-sphere/
maybe a little brittle when finished? Probably a little heavy. Not sure about getting a decent finish inside either. Ok, probably not a good idea.
Supersize hanging baskets as a frame work for the papermache?
http://www.hooksandlattice.com/xl-commercial-flower-basket.html
Could also use some fine garden mesh either wire or plastic around the ball with the papermache glue etc and make laminated layers, the mesh should help strengthen and stabilse the layers without to much extra weight.
Or chicken wire
Well that site leads me to an idea. The fiberglass inserts shown down the page. Those might work and they “seem to be” hemispheres, so two might be good enough for making a sphere. I don’t know that for sure, but they look like a possibility.
I think I am shooting at too big of a sphere. Well, I don't know, but I figured a big sphere would be needed for a light having 15,000 to 20,000 lumens, but I really don't know at all. I haven't a clue as to figure out just how big a sphere is necessary. Sorry to say, but it is all just a SWAG for me and most likely totally wrong.
I do know I want an 8" or 10" portal for the lights and I am also figuring a 2" portal for the sensor. If I use the "5% rule" for how much surface area can be used for portals, in a sphere, then I would be looking at a smaller sphere, to satisfy that requirement.
I did some calculations. Well, the Internet did some for me, as I can barely add 2+2 and come out with 5. Luckily, you can find most anything on the net.
So,
Surface area in Sq in, for different spheres, (rounded) and 5% of those numbers
16" - 804 40
24" - 1,809 90
30" - 2,827 141
36" - 4,071 203
Then I find the Sq in for the holes I might use and since I will use a 2" hole for the sensor, that will be consistent in all the results.
Sq in for holes plus a 2" hole
4" - 12 + 12 =24
6" - 28 + 12 =40
8" - 50 + 12 =62
10" - 78 + 12 =90
12" - 113 + 12 =125
So, if I want an 8" port plus a 2" sensor port, then I could get away with a 24" sphere and still fit within the 5% rule.
Maybe I am just going for too big, because there is stuff out there at the 24" size and it's a helluvalot cheaper than the 36" size.
Guess I need an Integrating Sphere professor to come tell me what I need, well, besides a $30,000 real integrating sphere.
Thanks for the experiment OL. May I suggest the song another one bites the dust for the start of the thread. 
I always liked Queen! :-)
OL, your calculation is correct :-) However, the build rules for an integrating sphere assume a very high reflective coating (like Spectralon). Since any DIY coating will be less reflective than that, I would be on the safe side, so an as big as possible sphere (but we know now that there's a limit to that), or a bit smaller port, say 7'' instead of 8.
This is the formula I will be using, or fairly close to it.
https://www.uleth.ca/phy/naylor/documents/pdf/2008_Noble_reflectance_characterization.pdf
Also, there's way too much info on Labsphere's tech library.
I’m new to the flashlight world here so please excuse a couple newbie questions. How perfect in shape must the inside of the IS be? And what surface finish would be acceptable? (speaking of the styro, not the coating on it). Does it need to be as smooth as a pre-formed styro sheet or would a little rougher do OK?
I ask because I’ve used a “Hot Knife” cutting styro for stucco work and that same principle could be applied to an electrically heated wire shaped into a perfect hemisphere rotated through a glued-up styro ‘box’ using a couple fixed pivot points. It would take a strong wire and a lot of current, but in theory this should work. With care you can sand styro quite smooth, especially using a shaped sanding block. Not quite perfect but pretty darn good.
Some of my crazy ideas work and some don’t. What do you think?
Phil
OOOOOOOOhhhhhh, this sounds interesting…. Hot wires, Styrofoam, sanding… Seriously, this sounds like a BUDGET winner with some figuring out on the technical side… I mean, bent wire and a couple of car batteries (or a buddies welder) , a jig to hold it and some alligator clips to provide the contacts on the wire, you could bend one end of the wire like a crank and connect the alligator clip near where the hemisphere bend starts, run the other end straight out for mounting in the jig and alligator clip the other lead near the other end of the hemisphere… I like the way you think Phil…
The welder might work. I’m guessing rather wildly but for a 3’ sphere it takes a ~5’ wire and with the thickness involved you’d need a kick-butt transformer like that. For anyone who tries this, do it outside because if the wire/blade is too hot there’s a huge amount of smoke and flames aren’t out of the question either! I do think you’ll need more than alligator clips, but the ground clamp and rod holder on the welder would certainly do OK.
They do make ‘hot wire’ foam cutters, but those hold the wire in tension and only cut straight. Otherwise this is the same principle.
Phil
I dont think he is carving the sphere from a solid piece of styro. I think the idea is to use the ball as a mold to form the sphere over, like the paper mache that fails. IIRC from other IS threads then they are cut in half, the ball removed and the paper halves glued back together.
Unless OL is trying something completely outside the sphere. I should probably read the thread, its grown somewhat and changed title since i last saw it.
The center part of a styro block cut like this would create a hemisphere. Make 2, glue together, then voila- 1 full sphere! Carpenter’s glue or even Elmers works great on styro so making a large enough ‘blank’ is easy and cheap. Sort of like my last GF 
Phil