3d printing some belt tensioners for my Ox… One more piece left to complete one side of the Y axis… I will let you know how it goes 
I tried these but they were not effective. The plastic sides flex too much. Ended up cutting some aluminum straps and then bolting together with the belt sandwiched. Doesn’t allow me to tension but have found them good so far.
@John_Matthews thanks for the input! Might have saved me several hours of print time. I printed at .1 layer height and 60% infill hoping they would be solid and offer no flex
Use PETG in your 3D prints if you want more rigidity.
@Alex_Krause do you have a link to the design files? I’m wondering if printing them on their side or at an angle might make them stronger (for the force direction in this application).
@SirGeekALot
http://www.thingiverse.com/thing:1293079 gt3 belt version
http://www.thingiverse.com/thing:1767676 Gt2 belt version
Love this file but when using double belts print two floating sets so you can pin the top belt to correct spacing to the bottom belt
as another data point - I have been using these on my OX - with intention of replacing them with something metal - but I have been using them for months. So long as you don’t over do it they work just fine.
I did what I printed mine (4x) over a week ago. I haven’t installed them yet (it is currently working fine, so I don’t want to mess with it). I plan to install them if/when I change the belts or see any signs of backlash.