I've been tossing around a similar idea with my Grand Am's trunk.
(Note: I have ZERO experience in any of this, just research and overthinking)
The plan is to build a basic metal rod framework cube on the trunk, probably using the spoiler mounting holes for the rear mounts. Then stack up foam layers on the outside of the box, working towards the rear glass. Do the other side, and the top. Shape it as needed, carve out more material from the inside, fiberglass it all, secure it to the trunk. Or something like that.
Can't wait to see how yours turns out.
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