I wanted to share my favorite trick from my build. The belt tensioning happens in the center shuttle or carriage. The belt catches on a toothed wedge, zip-tie style, while everything else, like the motor position, remains fixed. The narrow channel in the rear holds the other end of the belt folded over itself around a small rod; the cylindrical hole holds a stylus; and the fin activates an end stop. The picture is a cutaway view.
A possible improvement would be to find a place to tuck the tails of the belt after pulling them tight.
In the illustration the tension on the belt pulls up and to the right through the accurately positioned slot. This force slides the wedge further back into the slot, pressing down on and retaining the belt. The belt’s teeth are not rendered but face the teeth on the wedge.