The entire lid can be made of Fiberthane.
The material does flex, so an internal structure would be needed to give it strength and rigidity.
This can be done by creating bows inside the lid made from Fiberthane.
The end result would be a lightweight, strong, rigid, structure.
If you send me sketches I'd be glad to quote you on it. If we got a group order together we could amortize the design and CAD costs over the production run.
We could keep the individual sheets down in size by step cutting the top and making it out of two pieces.