Mine's a cabrio too - I just went out and messed with it a little more (in the rain this time!) and I opened the hatch, pulled the string tight, and then closed the hatch. Much better than the way I had it before. Thanks!
One of these days I may spring for a BikeBarn (
http://www.thebikebarn.net/smartbarn_model.html), but in the meantime if it lasts a couple of years this seems like a better alternative than my sunshade. I don't have a garage and live at about 6,000 feet 5 miles from the Mexican border. It's very sunny here.