What kind of host would this fit in? Just full-size Mags? Could I just buy a 3-cell Mag and put this in there or would there be other modifications required?
Something I've wondered about myself. Then remembered I had an LED dropin for the Ultrafire WF-500 (Which I must swap the LED in for a Q5 I have lying around) and on measuring it I think this may be intended for the WF-500 though the current draw will be fearsome and I doubt many protected 18650s will work with it. Probably need IMR cells for the best output.
Best bet - you'd get away with unprotected 18650s which are cheaper, but the IMRs are safer. I'm guessing that the 5 LED dropin is running 4-5 amps from the cells - runtimes will probably not be long. I doubt the claimed lumen ratings but it will be extremely bright. Probably a beam pattern like the Romisen RC-T6 which has a few beamshots scattered around this site.