I have been seriously thinking of making my own harness for a few years. I don't see why a T.M. couldn't be replicated.
The material used is only conveyor belt (which is cheap). You could use an existing T.M. and use that as a template. You could buy neoprene rolls, for padding, cheap. The webbing to keep it captive, would be cheap too.
I priced the raw materials out and the whole project could be done under $100 dollars.
I have not idea how long it would take though. I'm certain there would be a lot of trial and error.
If one were to make a custom harness, where would we go to test it and get it rated?