The Heavy Duty Pen Slider is nice, but a bit wobbly. I made this version which uses 1-4 LM3UU bearings (via the OpenSCAD file).

On my plotter, I ended up using 2 bearings and 1 hole, and leaving one hole without the bearing empty, as you can see on the picture. Otherwise the system is too constrained and the slider blocks.

You might have to enlarge the holes.