I found that the screw-together baseplates from the Gridfinity OpenSCAD generator tend to curl when screwed, as the bottom part gets compressed by the bolted joint.

I designed this extremelty simple shim that can be placed between the plates inside the bolted joint to compensate that compression. For me, 0.4mm thickness did the job the best, but I also included a 1mm version so that you can experiment with different thicknesses by scaling that model in the slicer.