I got some 1/2" and 3/4" copper foil from a guy I found on CPF.
It seems to work very well (way better than tin foil) and the 1/2 and 3/4 inch wide stuff fits the side of most dropins well.
Are you guys using similar stuff to get better heat transfer out of your XM-L's?
I'm going to order some more, it is so easy to work with and makes good smooth contact all the way around the inside edge of the lights.
I'd be interested to see how your using copper tape/foil in your lights. Please post any pictures you have. I use a thermal epoxy to fill around the emitter and encapsulate the driver board in the pill - when possible I fill the area between the reflector, emitter and casing/head as well. It makes the head virtually one solid piece which works well for the duty flashlights I build - very tough and excellent thermal transfer.
I remove/replace reflectors and entire dropins on a semi-regular basis, so thermal epoxy would not work for me. A lot of folks seem to use tin foil or even tin strips cut from soda cans to wrap the "flat" part of the reflector and make good contact with the side of the host.
Problem is, aluminum is not as good a conductor as copper, and the copper strips make much more consistent contact with both the dropin and the host because of their uniform thickness.
I bought the copper foil here
The copper foil is easily cut with regular scissors, it looks like this
I'm still looking for a cheap source of copper strips. I use soda can strips now, but I'd like to use copper, preferable something about 3 times as thick as soda can strips since I don't think any of my drop-ins need less than 3 layers of aluminum. I'm thinking copper roofing flashing might be the way to go but you have to buy it in big rolls that are hundreds of dollars (which may be what the CPF guy did). Or maybe an art supply place might have something.
I bought some adhesive copper foil and I think it is more adhesive than copper. It took quite a few layers and I'd like to minimize the layers as much as possible.
Edit; sorry I'm not very familiar with the units there, those are not cheap at all , here you can find it in those artisans suppliers.
http://shop.ebay.com/i.html?_from=R40&_trksid=m570&_nkw=copper+foil
Illumination Supply has been a reliable source for emitters and drivers when I needed them quickly and Craig's prices are very good. He has 12mm copper tape for $1 per foot. -Nutz
I think I spent a total of $7 on copper strip foil, and it was enough to do about 6 lights.
So at a buck per dropin, the expense is no big thing.
My local Hobby Store and Hardware stores have "hobby" selections of metals - but the pricing is very high. So unless I need it now - I'm still using the internet and snail mail - 
I've used his copper tape on every dropin I have. I wrap enough so that the dropin press-fits into the host. Heat transfer is almost immediate. Depending on the dropin, I'll sometimes use thermal grease, but find it almost un-necessary. It's a mess anyway.
Rich
I wound up getting a 6"x9" sheet of 5 mil copper on eBay from axvz for $4.50 shipped (I think that's US only). The aluminum can strips I'm using seem to be about 4 mil (0.10 mm). I think I'd still rather find a good source of 10 or 12 mil copper sheets. The 16 oz. flashing for roofing is 22 mil which would probably be too thick for most of my lights and maybe not easy to cut or bend into a nice cylinder.
That's a pretty good place. They have 10 mil that you linked to as well as 20 mil. They have 3" square samples for $4 shipped and 3" is just about perfect to go around a drop-in one time (you have to trim it a little). You could get 4-6 strips out of one sample. Hmmm . . . I'll wait until I get my 5 mil from eBay first.
Where can I get this in OZ?
The 5 mil 6"x9" copper sheet showed up yesterday and it turns out the supplier is in the Atlanta area (add them to 4Sevens and GoingGear). One edge got a ding in it, but they had it sandwiched between two pieces of cardboard so it is mostly in good shape and the edge was easy enough to smooth out. It seems barely thicker than strips from an aluminum can (I have a light that useds 3 layers of aluminum strips and it wouldn't accept 3 layers of copper; but I did get 2 layers of copper and 1 of aluminum to fit more snugly than the 3 aluminum strips).
The copper wasn't bright and shiny, but it wasn't tarnished, textured, coated, or anything either. Shows fingerprints like crazy. I was able to cut it with scissors just like aluminum strips, but the material seems a little softer, not as springy as aluminum can strips. If you are the only bidder, you can get a sheet for $4.50, otherwise $5.75 for Buy It Now.
I put the strips in a Solarforce L2p and it seems to work just fine with a XM-L drop-in, starting to get warm after 30 seconds or so. It might be good to get some different thicknesses so you could get a really good fit every time by combining a 5 mil and a 3 mil or a 10 mil and 5 mil to perfectly fill the gap. In general it seems like you want as few layers as possible.
Any one else use copper tape. I am thinking of buying a 30mm wide roll
The glue is your enemy... IF you do however buy it clean the glue off.
I bought some plain copper sheets and cut them into strips. I have some tape, but I agree that if you end up using multiple layers the adhesive will become a problem.
I'm bidding on a 9" x 12" 5mil copper sheet. That should be fine, right?
I won the auction. Paid $6 with free shipping from a US seller.
I have been using a role of the aluminum flue tape for doing duct work. This is not duct tape but the real metal foil tape that runs $15 or $20 a roll at the hardware stores. Works much better than aluminum foil as it doesnt have the air gaps and makes a nice clean wrap around the drop-in. 1 roll worth is probably several lifetimes worth of lights, luckily I use it for some other things so had a roll already.
Every additional layer adds two boundaries for heat to transfer across, so the fewer layers the better. With tape you are making a laminate of alternating layers of glue (insulator) and foil (conductor). This won’t be nearly as effective. (woo hoo 100th post!)
Some of the more expensive copper rolls have thermal glue to obviate the need for this step.