can't I just shape some polystrene foam between the bow and first bulkhead and again between the sterm and last buldhead and skip using float bags?

Yes, you can. It will be heavier than float bags and will likely accumulate gunk over time, but it can be done. When I was a kid, my father made a kayak that was like a skin-on-frame and he did as you are suggesting. Eventually ants made a nest in the foam and it got a bit waterlogged. Little bits of foam would float out of the ends whenever water got in the kayak. It was a bit of a mess, but it worked well enough.