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I'v biult a rainbow and a C, both work perfect. For a rainbow, i built the frame, and then on the top used two layers of 1x4, as apposed to 2x4s, and soaked them in water first, they bent perfectly. As for pvc, it just bent nicely using 1 1/2 inch pipe.
The C was a little more comlicated, i used 2 8' 2x4s, cut each in half, gave it a 7.5 degree cross cut on each, and attached them using playwood panels also cut with that angle. then just add feet at each of the intersections for support, and give a foot brace with a little more emphasis on the outside so it doesn't lean or tip over, and for pvc i used 2 inch pipe, and had a friend pull it to bend it as i screwed it, i screwed it from both ouside in and inside out to solidly hold it in place.
Rainbow: