Yet another avenue to explore would possibly be 3D printing with TPU flexible filament. I don't have a 3D printer but from what I read, having Googled it, it could work, though I suspect that you may have to experiment with the infill settings to obtain the required stiffness.
If I couldn't get direct replacements from a Cyclemaster spares specialist, like the one in Earls Shilton Liecester, I'd be tempted to buy a car engine or exhaust mount and carve it into shape, somehow.
John
Edited By John Hinkley on 30/01/2022 19:44:47