One of my runs has chicken wire over the top and I have a cheap silver tarp over that, attached by bungee cords. The roof is peaked and when we get rain or snow I reach up with a broom or whatever and "bounce" the accumulation off. The first year it was up we got enough snow that it broke the beam that you nail the side supports for the roof to, I don't know what its called, but it was a pain to replace without removing the roof. Its going on its 3rd or 4th year, has a few leaks but still works. The cheap blue ones aren't waterproof if the water sits on them. Trampoline tarps sound like a great idea, if they are available.