Hi Alan, when i build battery boxes, consider the cases, SLA batteries are a hard case so i fit them tight, NimH batteries are a soft case but hard cells, i fit these tight as well, Lipo batteries are delicate, soft cases and softish cells, protect the inside of the boxes, i line the boxes with foam paper type then cover with sticky back velvet like fablon, this gives a firm but soft hold to the cell packs.
Hope this helps
Mark