The STL is for GT2 belts, but the openSCAD file is included so you can modify it for other sizes.
Belt ends are looped through two printed blocks positioned so that they are within about 1". The belts are held in place using removable printed clips that force the belt's teeth to lock togoether when doubled over. Bolts are passed through the blocks, and the belt can be adjusted by tightening the bolts.
The design does not depend on tooth spacing (just that there are teeth). Only the belt height, width, and doubled height should need to be adjusted for other belt sizes.
2 x 1.25 or 1.5" #6 bolts
2 x #6 nuts
0) Print the parts with 0.4 layer height or less. Drill out the bolt holes if necessary.
1) Thread the belt through the clip, hook side up (see 1st picture).
2) Thread the belt through the block, so it is doubled over on itself (2nd picture). The ends should be coming out of the side with the indents (nut cut-outs).
3) Thread the doubled over belt through the clip. You may need to use a small flat-head screwdriver or similar to keep the ends together so they fit through the hole. Pull the belt tight on the block and push the clip forward until it snaps over the block.
4) Repeat steps 1 & 2 for the other side of the belt. Slide the block around so that it is within about 1" of the first block when you pull the belt tight.
5) Place two #6 nuts behind the holes on the first block. Put two bolts through the other block. Repeat step 3 for this block, and snap the 2nd clip in place.
6) Put the two pieces together and tighten the bolts evenly to tension the belt.
In theory zip ties can be used in place of the bolts?
Use customizer to adjust for other belt sizes. Alternatively, download the openscad file. In theory only the belt width, belt height, and belt doubled height should need to be changed if you are using #6 screws.