The fella I work for built his own. It uses aluminiuim bushes split in two, to clamp around the barrel without marking it. We have about 20 sets with different internal diametre for different profiles. The bushes are held by a two piece 440 clamp that clamps over the aluminium bush and has two studs either side to tighten the whole show down. Haven't seen a barrel yet that wouldn't succumb. This gives you an idea of what it looks like. Although his is taller and has man sized studs with nuts (only two)
The bushes can be milled to suit a tricky profile.
Sometimes a mandrel that fits the action like a bolt is needed.
Hope it gives you a starting point.
Good to see someone that doesn't want to use Stillsons or a Dawn vice. The amount of stuff we see with Stillson and Dawn vice marks is incredible and sometimes downright criminal.
If you find you have dug yourself a hole, the first thing to do is stop digging.
I just finished child-proofing our house - but they still get inside.