Make a fiberglass mold of your trunk lid where it mounts. Vacuum bag it.
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Then you need to build a welded frame out of aluminum that mounts on top of the mold you just made that that you can put studs in to mount it to your trunk.
if you don’t box in the fiberglass Duck Tail Spoiler it will warp and distort and pull away from the body and leave gaps.
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Hand laid fiberglass shrinks up like a Pringle and I’ve never bought a fiberglass panel that doesn’t need work to look right.
When you are done admire the work and craftsmanship. Because now your trunk lid will be so heavy you have to always hold it up with one hand.