Highland Hardware has some stone holders item #026461 that are made of chrome plated steel rods and rubber blocks. It is adjustable and will hold from 1.5" up to 10". And the rubber blocks make it pretty much non slip.

It's rust free so you canh keep the stones in it while you soak them.

I have seen others but this seems to hold the longest stones. Highlandhardware.com

John in Covina