Summary

Using Some GT2 belt (or else), 3 pulleys and 2 gantry Belt holders, this upgrade ensure that your Z axis will keep your X axis perfectly leveled from bottom to top. You can even adjust it.

Instructions

BOM first:

In addition to printed parts, you'd need:

1 meter GT2 5mm belt (approximatly 600mm will be used)
2 replacement m4 bolts at least 4mm longer than the originals
3 20mm m4 bolts (you can add the nuts too but the parts holes are thight enough to do without)
3 ball bearing (I used 624ZZ, the design allow for 1mm bigger/smaller...)
6 15mm/3mm wood screws
1 18mm/3mm wood screw (slightly longer is even better, I did not have one)

You'll also need to drill two big holes on the top and some small holes to help the wood screws to grip the wood where they are needed

The instructions

1) Remove the two top bolts, add the two top pulleys with the big holes pointing outward.
2) Secure them with the new, longer m4 bolts
3) Use the two big holes as a guide to drill same diameter (6mm) holes through the top.
4) Complete the two top pulleys parts with, for each, one bearing, 2 printed washers and one long m4 bolt
5) slide the belt through bot holes, ensuring the gripping side of the belt is turned up on the top of the printer
6) Attach the right extremity of the belt to the right holder, then, using the longest wood screw, fix the holder to the top right of the gantry, so that the pulley is perfectly aligned with the vertical rod.
7) Install the bottom pulley as on provided picture, ensure that from the side (left) view of the printer, it is centered to the vertical rod. Secure with 3 wood screws. Complete the pulley with the remaining bearing, two washers and the last long m4 bolt. Slide the belt under it (gripping side towards the bearing).
8) Secure the left holder on the side of the gantry with the last two remaing wood screws.
9) Move the hotend to the rightest position, and bring the gantry down by manualy actioning the Z coupler. Once the hotend will touch the bed, every extra turn you'll do to the coupler will bring the gantry closer to horizontal.
10)Try having the right side of the gantry approximatly hald a millimeter higher than the left.
11) Tension the belt and secure it to the right holder. A small flat screwdriver may help inserting the belt in the holder if necessary.
12) Using the coupler again, try to bring the gantry up. If everything went perfect, it will stabilize very close to horizontal.

Extra steps if necessary:

13) Check if the right part is too high or too low, repeat step 9 and adjust the belt accordingly in the right holder, 1 or 2 steps up or down. Repeat until leveling is reached. You may also try with a belt tensioner. or by adjusting the bed (not the best option).

OneUp TwoUp Z Axis upgrade for a perfectly horizontal X axis