foam under bulkheads

I am attaching plywood bulkheads to the hull of my boat to provide
sup****t for fuel tanks on a Morgan OI41.  Best info I have been able
to assemble says to put foam under the plywood before tabbing them to
hull.  Question is:  What sort of foam?  Thickness of foam?  Seems to
md that this application would require something denser than the
standard sheet foam available in home supply stores.  Any suggestions?