I bought a couple of Battlegrips from TangoDown
www.tangodownlss.com for $30 each. The arrived at my door in less than 24 hours which shocked the hell out of me.
I wanted something to store the DL-1 batteries for my Aimpoint and CR123A batteries for my TACM III light. My M-4 doesn't have a "trunk" in the buttstock to store them in. The Battlegrip has a duckbill removing the need for a "grapper".
I got one in standard tactical black for my Bushmaster and one in green for my Armalite. The green wasn't a very good match for the Armalite darker OD green. The Battlegrip is a couple shade of green too light for a good match but it matches better than a black one.
The Battlegrip will store two DL1's (Aimpoint) and either a pair of AA batteries (nightvision)or the CR123A (Surefire/TACM lights) using supplied foam spacers. When installed the things didn't rattle inside. The kit also included the pistol grip screw and a long shaft hex driver to match.
The bottom of the grip closes water tight with a plug that is retained in place very well. I've tried a couple other of these - the squeeze in rubber plug that seems to catch on stuff - the Battlegrip is recessed inside so there are no edges hanging out. The other storage I've tried is the one with the door that swings out - that thing never closed tight enough for me to bet $20 worth of life saving batteries inside.
The Battlegrip get's two thumbs up from me. This a a very good solution to the battery storage requirement on M-4 and CAR type rifles.