This is the final version of my X & Y tensioner. It is now a universal one usable on both Axis. It is slipped over the 4 bolts attaching the motor and utilizes the existing tension slots.
You will need to drill one hole for each tensioner. The Y uses a 20mm 3M Bolt and the X uses a 16mm one. The X can be a tight fit for thickness, so I suggest you try your first one on the X, if it's too thick you can still use it on the Y.
If you are looking for something that is more securely fastened down or you have issues with warping/pulling off the bolts, there are 2 other options that should fit the bill.
The first alternative uses countersunk bolts and was created by Tunell: http://www.thingiverse.com/thing:11775/
The second is the derivative of this one by M_G that adds a layer of plastic under bolt: http://www.thingiverse.com/thing:14098
Print, Clean, Check Bolt & Nut tolerances.
Just slip it over the bolts and screw into it.
You'll need to nearly remove the lower bolt from the front rail endcap, I suggest getting it flush with the nut. Slide it in between the front rail and the motor pulley and push it onto the bolts.