I have put a lot of effort and time into the design of this thing. Please be so kind and consider leaving a small tip if you decide to print and install this tensioner.
If the assembly of this appears too complicated to you and you'd prefer a tensioner that needs fewer extra parts, maybe this one is right for you: https://www.thingiverse.com/thing:3229130
UPDATE 1: I noticed that the main cause of friction when turning the wheel is indeed the PLA wheel rubbing against the PLA main body. This turned out to actually be the limiting factor to how much the belt could be tensioned instead of the belt tension itself. I've solved this by adding a washer into the main body and turning the wheel around. This way the nut contacts the washer and there is no contact between parts made of PLA. The wheel turns a lot easier after this modification! Now I am able to achieve ridiculous belt tension to the point that the belt sounds like a guitar string. Of course that is not the point but it's just nice to know it's possible :D
NOTE: The main body STL file prints without any supports! Turning on your bridging settings and removing the excess strings after printing is enough to achieve a great result!
This is a variant of my original design which performs better, but is more challenging to install and needs special parts. If you are willing to go the extra mile, the result will certainly satisfy you!
The parts needed are:
2 M4x45 lens head screws with flange (see image)
2 M4 threaded sleeve with 5 mm outter diameter (see image)
4 M4 Nuts
2 M4 Flange nuts (see image)
1 M6x20 hex bolt
1 M6 nut
1 M6x18x1.6 washer
The original M5 bolt and nut of the pulley are reusable.
For an installation guide, please see original design.
But note: The threaded sleeve must fit exactly through the guiderail! There can be absolutely no play or the tensioner will not function as intended. Adjust your horizontal expansion settings if necessary. If that is not enough and you can't edit STLs yourself, feel free to drop me a message and tell me how much i need to alter the size of the guiderail and I'll make a quick change to the file for you.