You should be able to slide the battery fore and aft to adjust the CG. Once you've established the proper CG, then the only time the battery would need to slide around is if you got a completely diffferent battery.

I'll tell you right now, gluing a balsa stop in isn't going to slow the battery down in a hard landing. It'll tear right through and take out the firewall again.

IMHO, the answer is foam rubber. Once you get the battery positioned, cut a block of foam to act as a bumper in front of the battery. That will absorb much of any impact, and keep the battery from sliding forward.