I just got through building a similar frame out of PVC about a week ago. I used three way Y's for the peak so that I had a vertical to support the load as well as rafters to the side. I then put a T on the stem of the Y at right angles and attached a ridge pole between the uprights. I solved the problem of diagonal bracing by using four guy ropes going back down to the stantion bases from the top of each frame. The frames were 3-4 feet apart. I used heavier tarps though, the blue ones are cheaper, but they usually shred in a couple of seasons. I was very pleased with the result, with the dodger down you can move around on the deck quite easily, and all hatches etc. are easy to get to. I uploaded a photo to my gallery in case you would like to take a look. Good luck, and don't breath the fumes from the PVC cement!
Nice construction you've done. How is it working out?
I'm planning on 1.5 inch PVC for the ridge pole, then 1" PCV coming down from it every 5 feet. They connect using a T that is 1.5 inches at the ends and 1" at the middle.
I'm also thinking about building-in an overhang, king of like the number "4", wjere the slanted part of the 4 keeps slanting up to the ridge pole. This is to keep the rain off the deck and allow ventilation.