A remix of the belt tensioner thing:690090 that uses 2 x MR63ZZ (3x6x2.5mm) ball bearings mounted on the stock pulley.
It makes for a more durable setup that prevents known issues with the stock tensioner, such as squeaking noises and non round circles, that typically develop after a number of printing hours.
With this design it's also significantly easier to insert and remove the pulley, while still keeping the exact same tension as the stock one.
Printed in ABS, at 0.2 layer height, 100% infill.
It's best printed with some supports for the initial overhangs. I used Simplify3D to add some manual supports, as shown in the last picture, but it should be also doable with automatically generated support structures.
You'll need 2 x MR63ZZ ball bearings like these. I found them on a local R/C hobby store. Note that ball bearings ticker than 2.5mm won't work because the pulley won't fit in the slot on the carriage.
More info on using ball bearings in the pulley on this Voltivo thread. You may also want to check this video out explaining how to install it.