OK, I’m brilliant.
I have a SCUBA regulator that’s missing the flow adjustment knob. Years and years ago (2006?) I bought a primary second stage for my open circuit regulator set from a shop in Calgary. I got it for $100 even though it was brand new because someone unscrewed the flow adjustment knob and then lost it. I’ve been diving it that way ever since because you could not buy that knob for love nor money.
Last night before I went to sleep I thought… “why not 3d print a knob?”.
The knob fits a square shaft 7mm x 7mm x 2mm and is sized to match the regulator. It attaches with a 3mm stainless hex bolt. Today I designed and printed the knob, and with a bit of drilling to open the bolt hole, it fits and works perfectly.