This is a derivative of the following file,

github.com/prusajr/PrusaMendel/blob/master/source/belt-clamp.scad

I replaced all hard coded dimensions with viariables, so now it is fully parametric.
It is configured for GT2 belts but you can make it fit any belt and also modify the clamp size.
The clamp is asymetric which means you can adjust your GT2 belt tension in 1mm steps(belt pitch / 2) by rotating the clamp 180deg.
The belt width has been resized from 7 to 10mm to allow more room for adjusment to line up the belt with the pulleys.
Thickness has been reduced from 7 to 3mm.