My timing belts were scooting over and rubbing against the side of the idler bearings. It wears out the belts and can make a howling sound.
The solution is to make sure the bearing is level, but I was having a hard time simultaneously tightening the screw, holding the nut, applying upward pressure to the bearing and making it perfectly level.
Others have machined solutions similar to this, or used mirror clamps. I wanted to try something I could print.
Note: On my kit, the holes in the melamine above my idler bearings are the same size as the holes for the idler bearings. I think some kits have larger holes above where the idler bearings go. If you have the larger holes there, the brackets might need to be sized differently.
Print 12 tensioner brackets and three spacers. Mine are ABS infill at 33% and they seem strong enough.
I replaced the 6-32 socket head screws holding the idler bearing with phillips head 6-32 1 3/4 inch screws because they are a bit longer. Same screws for the top brackets. The screws to pull the brackets together are 3/4 inch 6-32. My rostock max kit came with many extra washers and 6-32 lock nuts, so I'm using them.