Build the box to fit the battery. Make sure the box is glued to a decent bulkhead and some stringers. You want to spread out the support. And scrap sticks can bridge the box in back there. It really is easy. I did it at the field one afternoon with scrap wood from a crashed airplane and some CA. The guys all seemed amazed how simple it was and how little time it took. And the newbie who owned the airplane seemed to like the airplane more for the silly box than for how much better it flew. This is one amazing hobby.

The box I did for that one was really just a couple of support sticks. Or more like a cage. We actually used wireties to hold the battery to the framework so it wouldn't move.

The last one I did for myself was a box. And I CA'd a stick across the front to hold the battery in. There was a bit of rubber to keep the battery from moving at all, but I don't worry about vibration back that far.