I am about to start some custom fiberglass work on a '70s Marlin flat-bottom speed boat. We are going to build a cool, removable rear deck over the open cockpit. After giving the idea of creating a skin with 1/8" ply then forming some detail with foam board... I came across C-Flex, a reinforced fiberglass "board", 0.08 or 0.125" thick. Its normally used for one-off fiberglass construction.
I have no direct experience with C-Flex but have seen boats built of it and with parts made of it. It is used basically the same way plywood is, but is more expensive. Of course it has advantages that ply doesn't, such as it doesn't rot. You need to look at the cost/benefit comparison