I spent a couple of hours making some EAA tube clamps. These are the greatest things since sliced bread annd the modern flush toilet. I got them out of the EAA Welding book, and they work great.
The first picture shows all of the steel parts ready for welding/drilling/tapping/etc.
The second picture shows one in action with a piece of scrap tubing. The all thread you see has the wingnut welded to it and the angle was drilled and tapped for 1/4x20.
The third picture shows the back side of the clamp. There is a bolt that goes through the angle bits (its head is welded to the angle) and the wingnut allows you to easy loosen/tighten the rotatable angle bit.