Fiberglass can get pretty heavy...especially if you want it to be fairly strong.

What would definitely be lighter would be plastic....You might be able to talk a rotomolder to make some smaller pieces. The larger and more complex the mold, the more money it costs. The part itself will be relatively inexpensive...maybe $70 for a trunklid out of HDPE (although HDPE gets a little soft in the summer sun, plus you would have to match the plastic to the paint)
A plastic injected mold would be way too expensive.

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It is a truism that almost any sect, cult, or religion will legislate its creed into law if it acquires the political power to do so. Robert A. Heinlein


09 Jetta TDI
1985 300D