Ideas
I think it might be possible to get some left over 2" thick structural foam, and cut it to fit snuggly into the underside of the spoiler. I'd do 4-5 pieces (2 edges of the trunk, middle, and 2 more). Leave plenty of extra foam sticking out. Then cut and shave/file the exposed bottom of the foam blocks to fit the contour of the trunk lid. When done, you would attach with silicon adhesive.
You can make a sanding block to mimic the contour of any part of the trunk lip with bondo, plaster, etc, and some saran wrap to protect the paint. Use that to make a matching male piece, and use this piece with coarse sandpaper as a sanding form.
There are probably much easier ways, but this is what popped into my head first when I read your post.
Here's another ****eyed idea. Get 2 cans of that expanding foam insulation. This is the stuff that expands Line the surface of the trunk with saran wrap (well, do the whole rear end with that wide plastic packaging stuff). Then with a friend or 2, dispense the foam into the spoiler and quickly press it onto the trunk. You'll have to let the extra foam expand out the ends that overhang the rear fenders. You could do this in several steps so the interior of the spoiler is filled and you get a smooth, consistent bottom surface that matches the trunk. Then attach with double sided tape or silicon adhesive. You might have to trim and sand a bit to get rid of any squeeze out, but it should give you a fairly rigid surface that matches the car. The only question I have is if the foam would stick to the underside of the spoiler very well. If in doubt, you could glass in a few tabs with holes so the foam more of a mechanical lock into the underside of the spoiler.
In fact, I think I'll try this last idea with a long skinny test form (triangular cardboard shipping box from Fedex). Dow makes an expanding foam that is suitable to use in the wet, is black, and can be used in place of mortar "Pond and Stone"
John
