Here are four 2mm pitch GT2 belt clamp designs - two double ended and two single ended, with vertical and horizontal attachment holes sized to fit 6-32 screws.  The design is for 9.5 mm wide belt, but you can easily edit the rsdoc or stl files to change the height for 6mm wide belt, etc.  The horizontal-holed clamps like the design in the photo hold the smooth side of the belt 6.38 mm from the mounting surface.  You can also easily edit that distance.  The vertical-holed clamps hold the belt 2mm from the mounting surface (or turned over, flush with the surface), again, easily editable.

As you can see in the photo, the belt fits around the round post in each clamp then folds back on itself with the doubled thickness of the belt fitting tightly into the slots.  It won't slip no matter how much tension you put on the belt.

I don't know why but the 3D preview of the stl files shows dimensions incorrectly.  These parts are all 11.5 mm tall.  Some of the lengths are off, too.  Don't worry, the stl files are correct.

I printed solid in 0.2mm layers, no brim, no support.  I recommend ABS over PLA because the part can get exposed to heat in your printer and it will work better if it doesn't soften.