This is a very small and customizable belt tensioner with a very low profile on the belt movement.
By default the lost belt movement is about 23mm using a M3 screw and nut. However since this is customizable you can shrink it even more if you like. Same if you like the design and want a bigger more robust tensioner or more space for a bigger belt.
When using the default objects you need a M3 nut and a M3 10mm screw. Embed the nut into the matching hole and place the clamp over it. Now put in the screw from the other side. The belt goes in from the screw side, around the clamp and back to the side with the screw. The non teeth side of the belt should be facing outwards for optimal results and minimal stress on the belt itself.
By turning the screw the clamp will press against the clamp and push the belt outwards.
I used them on my TronXY X5 to tension the X and both Y belts. If printed right this tensioner is strong like hell.
FreeCAD source for customization is as always included.
Print with a brim, if you have issues with the bed adhesion.