Dyneema rope/climbing slings too, however I think billions of fibres behave differently to a solid lump. I had a quick look and several sites have it listed as suffering creep but that may be grade dependant. Possibly not a problem in this use as vertical guides.UHMWPE would probably be my choice, easy to machine and don't think it suffers from 'creep'
Pretty sure it's what synthetic winch rope is made from
Have also seen it used as body lift spacers on 4x4's and its what was used on the 4x4 picture I'm my avatar
I've not yet decided on which material yet, but the sizes I'll need (at the moment) are 8 blocks of 16 x 50 x 150.Used to use black nylatron in just that sort of application - blocks with a slot cut into them to use as a guides, wrapped around 50x50x5 RSA. And as solid blocks secured to run against hollow sections.
If you like the sound of oilon - I may have some going spare if you're not after massively thick or large pieces.