Replying to How to build a box with coping?

I want to build a box for my setup this spring/summer, but I want to go the whole 9 yards if possible, with an HDPE top instead of a bunch of little PVC pipes. But I'm wondering; how do you install the metal piping to the sides of the box that is so common on professionally built boxes like the ones you'd find in a real park? I know I can get pipe like that for not too much money, or I could even use PVC just for the coping, it'd probably work just as well; point is, how do I install it?