I was having difficulties tensioning the belts on my i3. So for the y-axis I created a new version of the idler with an integrated tensioner.
It was easier for me to just create the design from scratch although if requested I can do an openscad version.
Printed in PLA with .25 layers and 60% infill. The bridge is challenging but if your printer is tuned it shouldn't be too much of a problem.
I used Slic3r 0.9.9 to slice it which did a decent job too.
Parts - you'll want a M4 bolt for the idler - no different to before with a 624 bearing and bearing guides. For the tensioner I used a M3x20 bolt and two M3 nuts - one inside and one outside for locking the tension in place.
I've also added two nuts on the support rail so the idler is held in place but can still move when tensioning.
Edit: Changed the shape to octagonal to avoid extreme overhangs before big bridge.
y-idler...3mm - 3mm bearing hole for 623
y-idler...4mm - 4mm bearing hole for 624
Also: There seems to be a design very similar to this in the box_frame repository https://github.com/josefprusa/Prusa3/blob/master/box_frame/y-drivetrain.scad