Summary

Modified the original Railcore II rail alignment tool, so that it can be utilized to square the x- vs. y-axis.

Required tools:
Angle-iron

Instructions:
1) Print two of the extended rail alignment tools (ERAT).
2) Place the ERAT's over the front sides x-Rail, right to the front x-carrier.
3) Slide the y-carrier to the back.
4) Now you can comfortably sit an angle-iron on the ERAT's and against the y-rail.
5) Tension the belts so that the y-axis is perpendicular to the x-axis

Update (16. July 2019):
Removed material from the sides, so that the ERAT does not touch the belt anymore.