Help withy building new bench seat top

[pointguard]

Yes, the old one broke off and is now somewhere on the Garden State Parkway. What I have is a 32" x 24" x 16" fiberglass boat seat/cooler, swing back style. Posted a picture with the cushion.. So what I need to build is a new top. Is there a easy way to fabricate one out of fiberglass? Can I do this with all glass or do I need an inner core? The old one was all glass I believe. Thin, light and strong as I was able to stand on top with no problem. I weigh 170lbs. Anyway, any thoughts and help I would appreciate.