We used some over a pig pen several years ago. They do work. Main thing is to make sure they are out of the wind if placed horizontal. No matter how good you tie them they will come loose. It is best to have some kind of rafter to support the cloth across the span.
Farm Tek has some of the best ones and they can be custom ordered. Another good one is Sutherland Industries in Nebraska. They sew seatbelt webbing into the edges and the panels are really strong. I use those ones (4'x10' or 20') for a "poor man's" windbreak by tying them onto corral fences to provide wind protection in winter, but they could be used overhead too.
