I like Rubber Diaphragm sheet from OSH -- it is like flat inter-tube type material it is obtainable in a 1/8" thick sheet, -- you can use it for lots of things.
I make a rubber slip in
oil cooler block for my SPF and it works great. -- cranks the
oil temp up to 80-90C on a cold day. -- I need to get my "Wheat-a-Bix" box for the radiator portion though.