I think I'm going to build one of those. I think to make it more stable I will build it like this:
Picture two frames in a U shape spaced about 4' apart. The verticals would be maybe 3' or 4' apart as you view the U. These two frames would be connected with a long piece of PVC at the bottom left and bottom right of the U. Basically, if you look at the frontgate picture you see little horizontal pieces that are short. I'm saying that is one long piece. They're using two so that you can adjust the length I imagine, but having it connected will make it much more rigid. The hard part will be if they offer 3-way plumbing pieces at Home Depot. I suspect they do. I did an isometric sketch showing what I'm picturing but having trouble with the scanner.
I think if you built this with 1.5" PVC and kept it in the shade you'd probably be fine. Sand would add weight, but maybe too much. Let's see, if you had 2 @ 4' for the horizontals, 4 @ 2.5' for the verticals, and 2 @ 4' for the bottom of the U that would be 26' lf. I'll ignore wall thickness and say that 1.5" PVC has an area of 1.77 in^2/ft. So, that's 1.77/144*26 = .32 ft^3 which would be 100 pcf * .31 = 31#....hmmm, that's not bad, but it would warp like crazy if you picked it up with the sand. You'd probably have to drag it around. Still, this looks like a good solution.