Jeepman, if it's ok I'll throw my two cents in (Spaw's in the loo)
I started repacing the rubbers on my various capos 30 years ago. I probably own more that 60 of the little creatures by now, from antique ones to many of the modern "designer" ones.
I've tried wood, metal, leather, various different kinds of rubber, and even plastic. Although I certainly think that the Shubb is far and away the best 'QUICK' capo, I mostly use Dunlop 'c' clamp capos for most of my instruments. EVERY capo is carefully 'bent' to exactly fit the fingerboard of the instrument I'm using it on. I put it in a vise (without the rubber) and tap it repeatedly with a hammer until the bar is shaped properly. This helps give you more of that 'original' sound of your guitar, although you're still gonna lose sustain (as you do with fingers)
My favourite substance for 'rubber replacement' is 'floor tile'. Don't know exactly which brand, but the consistency of the rubber is perfect.
Also I cut VERY tiny 'v' shaped grooves in the rubber under where the 6th and 5th strings go (cuts down stretching, even further)
It seems like a lot of work, but I'm a fanatic about tuning.