For one that is operational and will handle a substantial impact, we used a scrap 12"-600# blind flange. It is supported by 1" galvanized condiut legs. The frame support is 5/8" rebar welded so that it has a v looking down for the pipe legs, and one piece about a foot long welded to the topside of the V to support the cross member. The flange is hanging from two pieces of an old tie down strap that we cut and singed the ends then bolted through two of the holes in the flange.
It gets some pretty nasty dents in it from some of the Allen mag loads but they can easily be ground down and painted over for a lasting target. I have to say however that this one is best left in the field if possible as it weighs a bunch.