The aftermarket f40 that Matt is talking about is I think made specifically for Capri.
Mine, I just measure the distance/width of the trunk for bolting.
Then I see how high I like it to be. I think mine is about 8" for medium.
I then go out to look for the spoiler that might fit.
Remember end of our trunk is rounded off.
I think I got lucky with mine.
If someone can remeasure the distance/width of the trunk, maybe I can help you search for one.