Yoga mats which are about 3/16" closed cell make good hatch gasket material. You do have to glue them in with contact cement. I have seen them in discount stores for $5 to $13 and thay are about 2 1/2' x 5'.

One suggestion - it you're doing a one piece gasket, do a wet contact cement job. If you let it dry and try to do a dry contact job, it's almost impossible to get the gasket in position evenly.