I usually just use the soft rubber casing from electrical wires. Some of my Aeg's have soft rubber casing and I just use that. I actually end up with great results. I have never tried anything else to be honest. I believe the softer the better when it comes to nubs. I dont think I would want to try anything hard like metal or whatever. I feel the nub should be able to flex and slightly squish I guess I could call it.