We made one some time back.. used blue styro to block it all in, glued into place and then finished the inside with epoxy resin and cloth. Turned out pretty good - used more foam to make two insulated lids (a small one for a quick beer grab, larger for loading and larger items.) It was a huge improvement on the "underhung' cooler that came with the boat.
1984 Fast/Nicholson 345 "FastForward"
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)