Summary

Improved y axis bracket belt tensioner with a screw adjustment.
Uses original mount screw locations. You will need longer screws.

Instructions

The belt goes through the slot on smaller piece around the "t" and and back through the other slot. Insert it into the large bracket which uses the same holes as the original and use a screw through the hole to tighten.
The large block is asymertical. The rectangular hole is off center to allow the belt to run perpendicular to the bed. Check alignment and reverse if needed.
I printed the small piece with support material.
Large piece printed with 30% fill.

Printrbot + y axis belt bracket tensioner