I designed this gantry cart to fit the Folgertech i3 2020 extrusions for the Z-Axis, this may work on other 2020 extrusions as well. The wheels do not fit OpenBuild V-Slots extrusions.
Needed to assemble (print the following)
3x Inner Support
2x Gantry Spacer
1x Gantry Eccentric Nut
3x M4 x 30mm bolts
3x M4 Nuts
1x M4 Washer (optional may help hold the eccentric nut on)
3x 608RS Bearings
A higher infill and/or 0.1mm resolution on the wheels may make them roll smoother, I sanded mine to get them nice and smooth.