Brian, Daniel Evans, and I made a field trip to Las Vegas today to examine how they run the Steel Challenge. Of particular interest was how their targets are put together.
Here is a typical Steel Challenge stage, in this case “5 to Go”.
This stage uses five 10” circles. Steel Challenge has four types of targets – 10” circle, 12” circle, and 12” square, and 18”x24” plates.
#6 Rebar makes up the stands for the smaller targets (8” and 10” circle plates and 12” plates). The are a tripod configuration with a longer back leg:
The tripod at the bottom has support brackets from scrap flat iron or rebar: 
A “T” is welded into the rebar at the right height, and a flat shelf welded to the back of the steel circles to hold the plate on the rebar
A #6 rebar sized hole lets the rebar stick through. The plates are easily removed from the stand.
The larger 18×24” plate targets use a different system with 3 parts. An H stand with a receiving pipe, a target stand pipe, and a plate with welded receiving cap:
An alternate approach would be to make 2 of the “tripod” stands and weld two support plates identical to those on the 10” circle plates onto the 18×24.
The steel used for plates is AR500 or AR400 steel. They report that the rebar handles bullet strikes fine and they haven’t had a stand break due to bullet hits.