I added M3 hex nut captures to the pieces in order to make installation and adjustment possible with only a single tool.
Instead of a single, universal, tensioner design, there are now four. I also added an extruded pad to the top tensioner pieces that help keep them positioned in the hole on the top of the tower.
The source files and STLs are not as nice as they could be, but I continue to be pleased with FreeCAD's ability to import STLs and convert them to usable solids.
The design is for M3 screws and nuts, but the FreeCAD source files are included in you would like to change things.
You will have a total of twelve tensioner pieces, three each of the four designs, and three spacers. Print the pieces and press the nuts into place. I've included a plater of all twelve tensioners that you will need. I didn't include the spacers as I found that different print settings gave better results, so I leave that up to you.
Install, adjust, and print with tight and straight belts!