Off and on I've been able to work on this in the hours after work and weekends sometimes. Here's a picture the new iteration as of today. 5 quite strong neodymium magnets hold it to the deck lid and two more hold supports underneath. This is the crap lousy rough fiberglass spoiler that I made by glassing over cardboard. In this version I soaked it in the tub until all the cardboard came off, then turned it upside down in an effort to prevent the curling that is made it useless so far, and used spray adhesive to attach some of the remnants of paper covered foam board to the bottom so as to prevent scratching from the fiberglass rubbing the paint.
The magnets are great, however, in my test drives I've noticed that at high speeds it will start to slide a millimeter or so. So I'm going to probably anchor one of the magnets with double-sided trim tape.
I think I'm going to call the whole setup "Hideous Ultralight" ...