Linear motion, cnc and cheap are a difficult combination.
Aint that the truth

I had promised my self not to post untill I actually start building again, but maybe I have an idea that might be worth exploring.

If the machine was solely for woodwork and the correct feeds and speeds could be achived with the rest of the system, then surley the loading on the bearings could be reduced and then a small nylon or delrin "Tyre" wouldnt be such a bad idea and allmost a cheap throw away maintainance part if wear occurs. (it could even have a rib that runs in the profile channel to proved some lateral restraint)

If the section was thin enough it shouldnt compress to much and if it was made wider than the bearing it could spead the load over a wider section on the ali profile.

Im not saying that it is a perfect solution but for this application and accuracy I'm sure it would be worth a try.

Later