I use plywood bulkheads (1/4 luan etc.) epoxy works well with rigid foam and I have done other projects with it. If you put a layer of cloth on each side, you can stand on it between two chairs. If you just epoxy coat it,will gain enormous strength but not as much as glassing. It's easy to cut and cheap and light and easy to get. I use wood for esthetics. Even then, many strip builders go through the effort of laying up a strip panel for use especially behind the seat where it can be seen.

Send me your mailing address and I'll send you the sheets on installing wood bulkheads and how I go about it.