I have an early Rostock Max v1 (just after the changeover from the side-mounted extruder). One of the quirks of the design are some idler pulleys at the top of the delta towers that are VERY difficult to get leveled correctly, in order to keep the belts from rubbing.
There's a neat solution, here: https://www.thingiverse.com/thing:87463, but I found that it's made for a version of the RMax v1 with different hole locations and sizes in the upper delta towers. It was an improvement, but I was still getting belt rubbing on the idlers and the aluminum extrusions of the towers themselves. The printer's 6 years old and I'm a little protective of it now so I wanted a better solution.
This bracket is designed to allow a greater length of throw for belt tension, and extremely fine-grained adjustment.
You will need to print 12 of these brackets, and will also want these parts:
18 6-32 x 3" bolts
36 6-32 washers
15 6-32 hex nuts
3 6-32 locknuts (with the teflon insert)
6 6-32 1/4" aluminum spacer sleeves (these go inside the holes on each side of the upper spacer)
3 6-32 1 1/2" (I think) aluminum spacer sleeves (these go on the upper bolt, in between the two MDF panels, above the idler pulley)
Washers go on the outside of each bolt, and hex nuts are used on every bolt end except the non-adjustable top bolt (with the spacer stack), where a locknut is used.
I printed these at 0.25 layer height with 50% infill in PLA, and they're rock solid. If you're worried about deflection of the lower (tension-bearing) axle bolt in the idler, you can try going up to a size 8 bolt for that; I thought about it but the 6 seemed to be working fine. There's a little deflection but it seems stable and is printing fine.