Need a little advice

I’ve got a friend who wants this modified into a rifle light http://www.fasttech.com/product/1359100-shadow-bl20-2cree-xm-l2-t6-4-mode-1600-lumen-led-b I intend to tap a second screw into the side to allow a rail interface. The cables getting shortened a bit and the battery pack converted into a two cell arrangement. Does anyone here know of a forward pistol grip that could hold the two 18650’s? I’d hate to have to run the cable all they way into the stock.

Maybe use 2 smaller cells in the grip instead of 18650s?