When using a MDF Y axis carriage that I bought as part of an e-bay kit, the mount holes for the heated print bed were 4mm wide. I noticed that even with 3mm holes, axis movement and the sawing action of the adjusting screws means that the holes became enlarged over time. This adds play to the Y axis and reduced quality prints.

This Y-Axis corner nut trap glues onto the wooden board and gives you a very precise mounting point for your heated print bed bolts. As it also traps a 3mm locking nut, it makes it easy to adjust the print bed height from above using a screw driver.

This design should also reduce the possibility of backlash originating from the mounting of the HPB

I have included the STL file that should produce a nice tight fit for the 3mm nuts.

The Scad file is also included. To use it put it in the same folder as the Prusa's source files as it takes its settings from there. I used Prusa's dimensions for the Timing belt mounts for the corner nut trap to make it all look matched.