This is a simple printable spacer to fill in the gap left when you move your stock POM leadscrew nuts onto an Oldham coupling. The spacers fit below the Oldham coupling creating a flush surface. 

They have a 10mm hole to allow the leadscrews through and 3.2mm screw holes spaced 18mm apart so you can re-use the stock threaded inserts in the gantry part. You should be able to screw your Oldham couplings in with M3x10.

Print 0.2mm layer height and at least 1mm (or 5x) top and bottom layers.