Downloaded Banholms belt tensioners (https://www.thingiverse.com/thing:1704100 and https://www.thingiverse.com/thing:1730521) and really liked them. Except that it was designed to pressure fit round boltheads into the tensioners. I figured that it would've been better to have it designed for a hex bolt, and after reading some comments it was clear that I wasn't the only one who had this thought.
So here's a remix of the tensioner modules but with slots for hex boltheads. I also threw in a turnwheel to hold a hexnut.
2x m4x40 hex bolt
2x m4 hex nut (preferably a locknut)
Usage is the same as Banholms original, you could just be a little more sure that the bolts won't turn as you tighten the belts