So these floppy belts need a sexy solution so I redesigned the component holding the sliding rail in place and the part to hold the pulley for the belt into one component with a tensioning mechanism and it will also add some more stability to the whole rig, I haven't printed it yet but I made a prototype and it seems to work well.

The idea is that a block will be placed inside the slot running along the side, this block will hold the pulley and a threaded rod will run out the back through another block I will glue in place and a nut at the back will allow me to tighten the belt, below I have added a rendering of the prototype I made as I forgot to take a picture of the working prototype.