Kris,
My top was in need of something when I got the trailer, previous owner(s) had installed a painted 'chip board' liner (yuck) and the former location of a roof vent was covered with a STOP sign.
After I got rid of all that junk, I realized the top sags if not supported with a rigid base/under-support. What I did, which could be improved upon, was to trim a sheet of 1/2 inch plywood to size, and installed shaped 'ribs' on the top to create a support for the
fiberglass that had slope - peak in middle sloping to 'no rib' at the edges. I made the fore and aft ribs shorter, so that it formed a kinda of a gable effect. I attempted to 'fill' the areas between the ribs with rigid foam, shaving it to the shape of the ribs.
The previous support had been bolted through the glass, with no apparent 'unintended consequences'. I filled the previous holes, and drilled new ones that were uniform and symetric. Bolted it up with stainless hardware, using sealant beneath the top washer, and aside from one area where a small puddle forms, it's worked like a charm for 3 years now.
If I had it to do again, I'd increase the height/slope of the pitch I created with the ribs, eliminating that one puddle.