Okay, hows about put the gantry on rollers? A pair of widely spaced ball bearings at either end riunning on a steel bar glued to the base, plus one sprung roller between each pair to hold it down.

To hold it square, a similar arrangement, turned through 90 degrees and close to the screw.

Wouldn't work so good for the Y but it fixes the big axis and gets your overall cost down.

You'd need some powerful springs which would want to bend the MDF, but an iron inclusion would fix that. Goodbye friction