I'll try to invoke yur imagination

Ah, OK, I think I get it... See attached pic. Getting the tolerances correct for a fit that would be able to be hand assenbled and disassembled without hurting the rotary encoder or other components is, I fear, out of my league...

So I'd still need a way to make sure the knob doesn't fall off the axle...
Then I think I might as well just make a section replacing the cut away material of the D section so that the axle is round again (attached with superglue or some sort of permanent LocTite) and go at it with a groove for an O-ring in the knob hole. That we'd probably be able to make work w/o too much trouble, as we're both used to working on/with paintball gear.

/Michael