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.