All of these Duck Tail spoilers look goofy after a while. Because they all look warped and who really wants to look at screw heads holding them on, the metal rusts where the holes are screwed in... It doesn't take much weight to overcome the springs for the trunk lids on these cars. So anything you do will make the spoiler more heavy than the old DC ones, and the trunk wont stay up.
It's easier to make a mold of your trunk lid with fiberglass, add your way of mounting it, studs or whatever to that. Then bond the spoiler to that. Put single back adhesive silicone strips between the spoiler and lid. Point is all of these will need a bottom to help them hold shape.
It's fiberglass. Most fiberglass parts need work for them to look right. They shrink, they warp, they twist right out of any mold.