Why can't you pull it down and attach it to the upright roll bar next to the seats? It doesn't need a flat surface to stretch over.
Edit: you can pick up short bungies at Walmart 4/$2 and hook them together with the supplied bungies if you need a little extra length....
It really won't cut the wind that much on the highway anyway... And if you really want to, you can just pick up some pet screen, grommets and bungies and make your own windscreen.... It's not hard, or expensive.