I posted this, rather pointlessly, to mouse's viola blog earlier: -

https://violaman.com/forum/for.....-10/#p2978

I think the biggest problem for me with the Bon Musica is that the flexibility of the metal body means that the friction between the feet and the violin is never enough, so that the rest is constantly moving about on the violin. The rest's body should be rigid, I feel, to prevent this.

I like the Kun and Artino expansion mechanisms (the Artino is more adjustable than the Kun). I have a need for the rest to extend over my shoulder (that was one of the original attractions of the Bon Musica), and you can achieve this, although the fastening nut becomes hard to access, but even there, since these rests are cheap, you can superglue the mechanism over your shoulder and just have the other foot adjustable.