[David Hodgson]

Whils the rubbers may appear good, all the forces on them are compression. There is a metal sleeve that determins how fare the nut tightens.
Still might pay to check the rubbers haven't compressed a bit allowing some fore/aft movement.
I couldn't source them either when i did mine. I adapted some others which I don't remember what they were off. This was to be temporary - 22 years ago.
I've been wanting to graft in a rack. Im courious how you did yours.