This is a remix of jwalbridge's excellent idler bracket for the Ender 3. My belt was dragging on the Y beam, and I wanted to free it up. I also wanted to print mine in PLA and discovered that it was a little weak with the tightness I had my belt set to.
All I've done is remix the existing part so that the cylinder the pulley rides on is extended and solid (added small reinforcement and pulley alignment to the base). Then I added a mirrored Female side which connects with the male side. I tested it on my Ender 3 and the setup is very strong and does not flex when I tighten the belt.
I created this with a tuned Ender 3. The fit is snug, but I didn't have to force the parts together. If anyone has a problem with fitment on an untuned Ender 3, I can expand the female side.
I didn't test it on the X-axis, but that may work as well. You may need the original "X_limit_switch bracket_mod" from jwalbridge.
One 5mm Bore (toothless or 20tooth) GT2 Idler Pulley
Optional: M5, 5mm longer Y-axis stepper motor bracket-to-Y-beam screw. This will allow it to extend through the male bracket and keep it firmly in place. One of the pictures shows me screwing that longer screw in with an allen key.
Infill can probably be lower, but at 80% grid the part was quite strong.