Torch mods and 3aaa flashlight mods commonly use 18500 or 14500's. Madvapes has a diagram and walkthrough on their site using an 18500. 18650s require a little more work to squeeze in the extra length but it has been done quite often.
Stacked CR2's will fit. The detonator
mod is a stacked cr2 5v regulated
mod on the same flashlight body. There are LONG youtube videos on how the detonator is put together.
The length on two stacked protected cr123's will give you some trouble, but you might be able to make it fit.
CR2's will have the shortest battery life and in my experience that battery life is too short.