This is a remix of the x-belt tensioner by freemark with a wider opening for using it with a GT2 pulley. I used a pulley with 16 teeth. In addition to the pulley you need one M3x20 screw and one M3 nut (better a lock nut) to mount the GT2 pulley to the printed part. And also two M3x25 screws (better thumb screws) and two more M3 nuts for mounting the part properly to the printer.
0.1 mm or slightly less ;-)
I used a 8 mm brim for a better print bed adhesion.