Ha HA!, Redtail, I have some of the same general design, except they are little kid's bicycle wheels (6", 8" and 12" across) and I boiled oak slats that bend around them all the way to pad the limb being bent. Yours are better, but I haven't been able to find any pipe like that where I can steal it. I occasionally see fiberglass or plastic stuff, but I want to be able to use heat.
I also found a small wire spool and screwed and glued the flat rounds on the ends into a form and drilled holes all along for the clamps to sit.