Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:2921900.
Updated 27th May 2018 I've added a beefed up version 2 of this that I recommend you to use in place of the previous version. This should remove any flexing from the previous one if you've had any problems. (I havent but just to be sure)
This is a part I made and have tested for quite a while with great success, it quickly straightens the belts to make everything nice and square, as it should be.
The part fits at the back using the guide wheel screws which are plenty long enough for this.
Use at least 3-4 perimeters and 100% infill.
The part may seem slightly flexible before fitting but after assembly will be much more rigid as the screws are very close to the ends.