I used West system with the 209 super slow hardener.

I got disappointing results. Better than raw steel tubing perhaps but nothing as good as some people report / claim.

It seems you need quite a thick pour (5mm plus) to overcome surface tension.
Epoxy is not a stiff material, so now you have 5mm of relatively soft stuff between your rails and the underlying metal.

I got indents from my rails in the epoxy despite waiting weeks for it to harden before mounting rails.

Overall I would not go for epoxy again.

There is no good cheap solution.

Send out for machining - $$$$$$$

Making the frame very adjustable is probably the best option. You can then do a lot with a machinists level and a good straight edge.