Richard if all else fails (and I sincerely hope it doesn't!), you could always use a white nylon type material called "Delrin".I have had to machine bushes/ bearings for many years out of this stuff. It is very hard wearing and easily machined. I don't know if I have any off cuts stashed away but I will certainly look now. If you could supply dimensions I would gladly knock up a couple for you to try.
HTH if you are still stuck
Ian 👍
PS. I am not familiar with the XBR setup but is it possible to drill/tap the spindle and fit a grease nipple?