Here are some close-up photos of one of the Trangle trees I built.
They weren't built really with any precision or specific plans--just a bit of measuring and mostly eyeballing. I left the legs on the bottom for a bit of a snow allowance. As for the angles: 30 degrees on top and 75 degrees on the bottom of each side.
I nailed the frame together, got some hinges, and screwed them into the cross-member and the third support leg (as you can see in one of my photos, the holes I pre-drilled were a bit too small... oops). I then painted them green. I also drilled holes in the bottom of each leg to pass an eye bolt through so I could stake it into the ground.
As for getting the lights on, I didn't wrap them for two reasons: 1) It would be a pain, and 2) I didn't have very many lights to use, (and why have lights on the back where people wouldn't see them?), so what I did was place all the lights on the front. I placed each tree face up flat on the floor, then I stretched my red, green, and white light strings on top using twist ties to hold them temporarily, "massaged" the bulbs in a way to minimize the number of them buried underneath the wires, and when I was happy with it, I zip tied at all down nice and snug. Then, I zig-zagged my multi-color lights wrapping around it all to "fill" in the trees' insides; they didn't slip around as the border lights provided the "grip".