MiniCel Foam, 0.75 or 3 inches thick, sold in 12" x 24" and 24" x 24" blocks. Grey flexible closed cell foam for bulkheads. Easy to cut and shape with a serrated knife or wood saw. Glue with 3M-5200 sealant. One block is enough for two bulkheads in most kayaks. Also good for carved seat bottoms.  Here are some ideas for fitting out cockpits. 

This premium foam is formulated specifically for marine use.  It has a very long life and will NOT absorb water.