I used a material sold for that purpose at "west marine".
It it's supposed to be closed cell foam, it is black in color,
about 1/4" thick & it's sticky on one side.

DO NOT USE IT !!

it isn't closed cell ! it's open cell but the cells are so small you cant see them. I only realised it when I had leaks & I started looking & I realised that
lousy material absorbs water.

It's glue doesn't hold !

It costs about 4$

I converted to regular minicell foam, around 1/4" thick.
I already glued it with contact cement but didn't had the chance
to test it.

good luck,

Erez

: Can anyone recomend material type and thickness for sealing hatch on
: a custom bulkhead hatch.
: A source would be great help.
: thanks,
: peter