When I used to have a C-West wing on my car, we used a thin foam strip (about 7-8mm in width and 5mm thick) which was sticky on one side, to seal it against the boot lid.. Stick it all around the edge of the base of the wing, and it seals perfectly when its bolted down.

I think it came in rolls and I have no idea where to get it from, but try a place that sells spoilers and body kits.. or Bunnings might have something similar.