Thanks for all the input guys.
I have 3 designs now to test out over the next few weeks. 2x designs come from the HO world and the third is the MTH “kinematic” coupler style in O scale.
I intend to make them either a replacement or an upgrade to the standard coupling, not a rigid and hard to un-couple type.
Further my aim is to make the diaphragms almost touch when straight and extend in corners so nothing fouls. Obviously the design must consider the length of the carriage, the wheel base of the bogie (truck), the current mounting point and length of current coupling, the minimum radius or diameter of the curves on your layout etc... Sounds like fun!
Cheers Pete