If it's a marching baritone, I think something like this would work (forgive the primitive drawing)
The idea is to use straps to hold a plate over the player's sternum (less affected by breathing than the abdomen, less affected by marching than the belt-buckle region). Attach some sort of rod to the baritone to support the weight, so that the left hand just "swings" it up to the embouchure. The attachment at the horn should pivot forward and back, but not side to side. That will also allow the player to do a "horns up/horns down" movement, and allow some control over the pressure at the chops.
At the other end, it should "plug into" the plate, but the plate should limit the forward/downward movement of the rod. Some assistance may be requred to get the horn attached to the plate, but from there on the player should do fine.
Good luck!
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Joe Baker, who admires people who won't let disabilities disable them.
