Summary

This Belt Tensioner can be adjusted with a set hex screw and consists of two parts. Designed for the Ultimaker, but this thing should work on other machines/belts too.

Features:

adjustable
small, doesn't block mechanics
you only need a M4 set screw for assembly

Instructions

Orientations are Ultimaker-specific. For other machines this will be different.

Print the two parts, no support material needed. Should have at least 50 % infill for good rigidity. ( I printed mine with 100% infill, this doesn't take significantly longer)
Screw a M4 set screw (there should be some coming with your ultimaker) in the block until it almost sticks out at the inner side
Push the block onto your belt with the two arms facing inwards. The screw head should face down for left/right belt and up for front/rear belts.
Now hold the block with one hand and push in the little inset with your other hand with the flat side facing to the belt.
Check if the tensioner hits your sliding blocks. If so, you might consider (re-)aligning your pulleys and or shifting the tensioner a little bit to the outside.
Screw in the inset screw to tighten your belt.

Have fun! :-)

Adjustable Belt Tensioner