Although this isn’t a hardware modification (yet), here is how I tension the Y belt.
- Loosen the bolts on the Y idler
- Pull the idler out until the belt is under the amount of tension you require.
- Tighten the inside m5 bolts to clamp the Y idler in place.
- Tighten the other bolts, done.
To make this easier it might be good to have the m5 bolts on the end of the T slot have nyloc nuts or double nuts to hold the bolts loosely on Y idler. Then rather than pulling the idler in step 2 you could set the tension by those screws.
If anyone has a better tensioning method, let me know!