I didn't, I just used the plain 3mm but.... I did spend a lot of time going over the fibreglass to remove all traces of old glue beforehand and even used thinners to get off any small traces. The surface was smooth to touch although of course there's slight undulations but it's basically smooth.
It's a while ago now but I think the original was probably thicker, probably 1/4" back in the 70s. You could recreate this because you can buy 3mm foam and make up a sandwich using a thin layer of spray adhesive but I didn't bother.
Brian