The only advantage of closed cell foam bulkheads is installation. You can cut out a shape that's a bit larger than the opening and press it in with some calk/adhesive and you have a bulkhead. 3" thick closed cell is not always locally available and it's expensive. The practice of using closed cell for bulkheads came with plastic kayaks where they didn't have a lot of choices.

You can make plywood bulkheads and coat them with epoxy and install them with epoxy fillets. 1/8" or 1/4" in fine. Precoat them first. If you kayak is done, you can insert them and put the filets in working from the hatch openings and use a bent knife or glue brush on a stick in the cockpit side. I find that glassing is not necessary.