Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:4099368.

I created this simple piece to help align MGN12 linear rails into the centre of 2020 aluminium extrusions. I have rails coming in the mail so I haven't tested it yet but I used dimensions from the Hiwin website and the extrusion part fits so, in theory, it should fit. Please leave a comment if it does not fit.

You can print 2 and put one either end of the rail to centre it.