What you are looking for is not the standard blue tarp but the roll of material that they use in construction and is UV stabilized
http://www.mpdplastics.com/string.html
with that system you would make the frame and tack or staple the tarp to it with strips
the other method, that we use for boats here in Alaska is a shrinkable material that you lay out, and then use a heater to shrink into shape, works very well you would make your supporting frame to get the run off pitch lay out the material *you can join multiple pieces together, and then shrink the item to make a snug fit i
http://www.boatshrink.com/index.htm
Sharyn