The rubber assembly is the round ways frame bracket and supports the left spindle head pivot point. Front and rear rubber positions are designed for 3/4" square rubbers that contact the rosette cams mid center of the rubber or 3/8" above the rubber platform. Any size rubber can be used so long as the rosette cam contact point can be shimmed to 3/8" height over the rubber platform.
We are using Ultra High Molecular Weight Polyethylene (UHMWPE) for rubbers. It's almost as slick as Teflon and cuts and machines very nicely with sharp tools. We get it in 3/4" square bars and cut it to 4" lengths and whatever end shape needed.
We ordered last from http://www.mcmaster.com for $2.46 per linear foot. The Part Number is 8702K111, Polyethylene (UHMW) Rectangular Bar 3/4" Thick, 3/4" Wide.
"Ultra high molecular weight polyethylene, is a very tough material, with the highest impact strength of any thermoplastic presently made." "It has a very low coefficient of friction, is self-lubricating, and is highly resistant to abrasion (15 times more resistant to abrasion than carbon steel). Its coefficient of friction is" ... "comparable to that of Teflon, but UHMWPE has better abrasion resistance than Teflon. It is odorless, tasteless, and nontoxic." (quotes from Wikipedia)
This seems to me like the very best material to use for a friction rubber since it is so easily machined and has such a low coefficient of friction which results in negligible wear to the rosette cams and the rubber itself. Also, the price of $0.62 each plus shipping for a simple rubber is difficult to beat.
We plan to make a ball bearing rubber and, expect it would have the lowest friction of anything we could use if the radius of the ball bearing did not alter the result we were trying to obtain from the rosette pattern.