I think I'll just buy a 1/2" thick piece of aluminum as long as the head and drill holes to bolt it to the head. I'm thinking just a bead of sealant would be good and bolt it right to the head? Do you really need a gasket for this?
I'd use a gasket, less messy than dealing with some kind of sealant.
You can buy gasket material in sheets or rolls.
I use 1/16 thick paper/rubber type gasket material.
I contact glue the gasket material to the test plate.
I've seen thin rubber sheet glued to test plates as well.
That way it's always ready to go, just bolt it on, no messin' with sealant.