I wanted to be able to tension my Y belt without having to work underneath the machine, so I decided to try adding a tensioner to the Y end idler. I have this mounted on my MendelMax and it seems to work very well. In fact you have to be careful not to over tighten it.
Edit, March 1, 2012
I fixed the mounts with the ridge by eliminating the ridge around and above the hole.
I designed this part to use the same screws as the rest of the MendelMax. It requires 3 of the M5 20mm screws and 5 of the M5 flat nuts, so if you bought the bags of 100 you have lots of them left over, 2 washers and 2 of the M5 10mm screws for mounting. You will also need parts to install the 2 608 bearings. I used a 2" 5/16 bolt and 2 nuts. This length gave me enough non-threaded section to have the bearings ride on and it allowed me to put on a second locking nut. You could also use a bit of the leftover 8mm rod and just compression fit it in, or you can use some of your leftover 8mm or 5/16 drill rod and some nuts. I designed the idler to fit the 2 bearings and 2 washers between the bearings and the plastic. I designed the mount pieces with a ridge that fits into the slot of the extrusion and that is the version that I printed and am using on my printer, but I don't think that they are necessary, so I posted files of the mounts with and without the ridge.