Carsten Werner from Köln (Germany) - also a right below elbow amputee - allowed me to web publish his design and implementation for a plectrum mount. With that, he passed the entry test for a Jazz College back in 1988. And with all due respect for other one handed guitar players, that is an achievement I find hard to match.

The design contains a round piece of metal with one part bent up straight. Against that, a metal bar is squeezed with two screws. The rest - semi circular slit for rotationally variant mounting - is self evident from the photographs. In terms of positioning on the socket, it appears to be necessary to place the plectrum close to the end of the radius bone - or wherever else you feel most comfortable.