Has anybody used the clear vinyl 'fabric' that's available at some craft stores for a tarp? A lot of it is pretty heavy stuff, but it can be found in surprisingly light weights if you dig around. I can't find specific weights online, but I felt some that I think was 8 gauge at the fabric store a few days ago and thought it might be doable, if a little heavy.
I'm just not positive about the durability if it's stretched. It is absolutely window-glass clear, especially when under a little tension. It would certainly blow Cuben out of the water in that regard!