I made one of these great sleeve cases for my Raspberry Pi B+.
I wanted to be able to mount it on my Printrbot Simple Metal, so I started by making a device to attach it easily to the PSM. Using sketchup and a bit of trial and error I came up with a bracket that would clip onto the bottom of the two 12mm Z-axis rods.
The second piece is just a simple saddle clamp for the Pi case. It hangs off the bracket. you may want to print two of the PiClip.stl and you can position them however you like on the Pi case, depending on what ports you require access to.
Please note the bracket is 5 mm thick. Be sure you have sufficient clearance on your PSM when the Z-axis is homed. Mine has approx 7-8mm of clearance there and this bracket works fine.