Halloween // Advice on making a bier/casket stand strong enough to lie down on

I cannot for the life of me find any tutorials for making what I hitherto called a plinth or altar—the bier. I do cosplay and need a slab-looking thing to pose on, but I am limited in my inability to use power saws and, lack of floor space (narrow, awkward garage).

I can’t cut plywood—tried it, my line was crooked af and even trying to stand it together to drill was impossible as I have almost no flat floor space to keep things straight.

I was wondering if insulation foam would be strong enough. Or if I would need to get some 1x3s and reinforce it from the inside. I am not a very heavy person and it doesn’t need to last, so if nails, glue, and rigid foam will do the job, I’ll go with that.