I'm glad to say I almost never do these anymore.
But for those of you who want/have to...
If you have a router table, it's really easy to make good hoop platens... Drill a hole in the middle of a square the size of the circle you want, and put a matching size nail half of the diameter away from a straight cut bit.
All you have to do is rotate it, and you have a "perfect" circle. (well, closer than you can cut with a saber saw, anyway.) Bondo the hole, slap it on your bracket or board, and you're rolling.
Those hoops are pretty unforgiving of bumps--you'll get areas that don't grip, which is no fun when you're flashing. If you want to get real fancy, gluing a strip of thin cork or rubber around the edge will help the hoop grip better.
