[RidgeRunner]

Core = Plywood. If you go to the trouble of building a mold what you make is the female version so when you lay the glass in you are really making the topside of your lid. Like they build hulls from molds. Gelcoat,Glass,stringers or core, More glass. You might want to look into some of the ready made fiberglass or composite panels. I never priced them but I have seen them online. Then you need only worry about sealing the edges and fitment. Depending on what you can find you could use some fiberglass tape and a minimal amount of materials.