custom guide for 3d printer or router

[Greeny]

If you aren't sure and don't have a good means of determining that any rail is straight then you're guessing and are thus stuffed anyway, likewise level and square are a guess unless you measure them.

There is a tried, tested, simple and cheap way of aligning and levelling v-rail using a tensioned wire along the apex of the rail.
Methods for this are given in great detail on the mechmate forum i linked to

Bent rails can be straightened if you have means to measure errors in straightness, level and square can also be adjusted if you have the required metrology.

The op has already stated that the budget is tight, too tight for supported rails, so i was guessing it would also be too tight for the 'required metrology'. Besides, even if the aluminium square tube could be straightened its still a terrible design.

Vulnerability to contamination is to a large extent a design issue.
- Nick

Exactly, which is why i suggested looking at an alternate budget design that is proven to have low vunerability to contamination.