The Y-axis belts on my Replicator* have always been a bit floppy, and attempting to tighten them a notch always seemed to skew the carriage and make them drum tight. The original source Thing:30708 seemed like a good idea. There were a couple of issues that needed to be fixed up though. First, I try to never leave a free nut that has to be placed and tightened blindly - a printed-in nut holder is always the way to go. Secondly, you would have to carefully measure and drill the holes to ensure the thing is correctly aligned with the belt. This was solved by making it clip over the edge of the top plate, so that the edge of the plate automatically aligns it properly. Then only one mounting screw is required. Also, the clip itself is the drill guide - you can't miss.
The clip is symmetric so you can just print two of the same for the left and right belts on the Replicator. This should work for Replicator clones too, though I think many of them only use one belt.
It could be reworked to place it closer to the drive pully, but it seems fine where it is. Also you may need to alter the OpenSCAD file if your top plate thickness is significantly different from 6mm.
*(Yes Replicator, not Replicator 1, or Original Replicator, but Replicator, period. If MakerBot marketing geniuses wanted to create confusion by also naming a later model 'Replicator' that's a problem of their making, and I don't have to cater to it.)
Yeah, I printed it so that there is a bunch of support between the top and bottom of the clip. It comes out easily enough though.
It'll need an M3 nut placed in the holder, and an M3-16 bolt to mount the clip. An M3-25 or 30 is needed for the idler axle.
The dimensions of the original thing looked like a good fit, so I imported it's stl into OpenSCAD and designed over it. I placed my objects to match the original principal features. I do this often when I want to alter a thing that doesn't provide the CAD source.