I made a shoulder rig (just to see if I could

) based on the WW2 USGI Tanker Holster.

Not a concealment rig with the strap going across your chest.
But it is modular and fits a large number of my big FBs
Back in the 80's Gerber made a shoulder rig for their MK1.
It was a large 2" strip of elastic, much like a suspender strap, with velcro on both ends.
It went around your shoulders in a figure 8.
The sheath attachment had a back panel of stiff leather that the strap threaded through. It was covered in velcro.
The knife sheath had a matching back of velcro, making it a bit adjustable.
The downside was that the handle of the inverted knife was down around the belt line making it difficult to grab.
Dozier makes a concealment rig for his NY Special that's simplicity itself.
Hope that helps.