I printed the design from donnyb99, but the tensioner is a little bit fragile, not easy to print and you need to disassemble the belt.
So I tried to optimize it with this design. The knob is from his tensioner, looks good :-)
You have to turn the two models "Anschlag" and "WangeX" 180°, I forgot to orientate them correctly for printing.
Update: I uploaded the two parts correctly orientated.
Use a plier to remove the side-supports.
You also need to remove a little corner (2x2mm), which collides with the sheet metall. I hope you can see them on the third picture. Sorry :-)
You will need:
1x GT2 pulley 20 teeth with bearing (bore 5mm)
1x DIN933 M5x30
2x DIN934 M5
2x DIN934 M3
2x DIN912 M3x20
1x DIN912 M5x20
I used Autodesk Inventor to design the system.