I've been having a hard time finding a zoomable host and reviews of zoomable hosts.
I am looking for a good zoomable host to build a DIY light with.
Plan on running a XM-L2 and hoping to use a 2.8amp or 3.0amp driver, so it needs to adequately vent heat.
Powered by 1 (maybe 2) 18650.
I would be willing to mod a pre-built light with a new driver and emitter.
Can anyone recommend a good zoomable host?
There's a bunch out there, but one of the best for the money is the UF-T20, one place to get one is here: manafont uniquefire UF-T20. Zoomies are just not that popular, you could say for good reason - they do a few things ok instead of doing one thing well. I like them but you lose a lot of lumens with typical aspherics, and throw is not as good as you might think, compared to reflector throwers.
In a host format, yes, hard to find, specially one that's been reviewed. This one looks pretty good for a very good price: http://www.cnqualitygoods.com/goods.php?id=1957
I also like the T-20 but with a 3amp driver and a de-domed XP-G2 on copper. It makes a great light for throw and the flood is a nice tint and very full. I’m unusual around here because I like zoomies.
I only like them as far as I can throw them.
I used to be a pitcher on a baseball team. I can throw one of them compact little buggars pretty far. 
I was reading this thread about de-doming and it says that
A sad observation is that you loose a lot of light between the emitter and the outside world in a aspheric, even when zoomed out it looks already like around 40%, and zoomed in 60%.
So I am losing almost half of my brightness because of the aspheric lens?
Yes you lose quite a bit with an aspheric not a lot with the de-doming though and if you want throw more than flood it’s the way to go. The thing about the t-20 is that it has a lot of travel so you lose a little less than you do with most of them and it has a good enough pill that the heat transfer is pretty good. If what you are looking for is just a ton of lumens in a small light then something like the C8 is a good choice. Lightmalls XM-L2 for instance puts out a ton of light for around $20.
I won't be de-doming for a while, this is going to be my first build.
Since I loose all that light, think I would be better off making a good pocket thrower?
De-doming is the easiest part of the mod. All you do is soak it in gasoline and wait a few hours and the dome comes right off. After that you just have to be careful not to bump the little wires on the emitter. A de-domed emitter will give you much more throw especially in a pocket sized light. By throw I mean distance, if you want a lot of distance you de-dome. If you want flood, close range brightness then don’t de-dome.
Either way if you want to maximize total lumens then go for a reflector and not a aspheric. A C8 is just a great pocket 18650 light everyone should have one and most of us do have one. I highly suggest a C8 for a first mod or first light period. The convoy line of light at Fast Tech are great C8’s too and you can simply pick and choose what you want.
If you are modding for the fun of putting it together or for the experience you can start with this host. They also have all the various parts to build it. Except copper emitters but you don’t need that unless you are going with a lot of amps.
http://www.fasttech.com/product/1277302-convoy-c8-diy-led-flashlight-host
Of course, the SK68 is zoomable, you didn’t specify a size, as far as I can see. In 18650 size, there are SK98 or scaled up SK68 clones, like the Raysoon TD-398 Review: Raysoon TD-398 (XM-L T6 | 18650). These have good cooling, except that the one linked above has a hollow pill, unlike a Sipik SK68.
I am planning my first emitter swap to be a large lensed SK68 clone three mode.
About light loss, there is a new design, apparently intended for mass production: 3-mode Zoom AA/1450 EXPIRED ! "Black" $1.99 Max 3ea. It is way dimmer than an SK68, with maybe 80% or 90% of the current, because it is slim and the lens has a long focal length. I put together a Fresnel lens light that has less loss because it has a higher size to focal length ratio.
Another vote for the UF-T20 here. I have an XM-L2 on copper driven at 3A in mine.
What are the advantages to de-doming with an aspheric?
Op, have you considered TIR optics at all?
The host looks nice but I find it silly to see the finning on the slider, how does the heat get there? If they just leave the fins out and make a thinner flashlight...
More throw. I think TomE said his went from about 75kcd to 125Kcd when he de-domed it. My T-20 is hitting 110Kcd put I’m not driving mine as hard as he is.
Yes - that was the increase, roughly. Usually you get a little over double kcd when de-doming, but with XP-G2's it's been somewhat less, but I think it's because I can't seem to get the same high amps after de-doming. I've noticed it, as well as RacerR86, maybe others. I don't think the "lower than expected" result had anything to do with the aspheric lens. Basically de-doming works about the same with an aspheric as it does with a reflector light - tint shift, ~20% loss of lumens, about double the kcd, tighter beam.