Not knowing what your robot looks like in the area you intend to mount it, there's really no way to say what would work well. You need to check all 29 directions it could move to make sure it can't move that way. 26 directions are translations:
- toward each of the 6 faces of the battery
- toward each of the 8 corners
- toward each of the 12 edges
And the other three are rotations: roll, pitch, yaw.
If you're using the KoP chassis and have enough room inside, a battery fits snugly inside the front or rear plate. That will knock out roll, pitch, and five more directions by itself, and give support for several of the others.