I hope I've done this right.
I printed the;
"Case for RepRapDiscount Smart Controller (http://www.thingiverse.com/thing:35438) by wersy"
And I wanted to mount it on the top rail of my Sunhokey Prusa i3 Printer, so I created this.
The two holes on each that mount to the case are 2.5 diameter so they can be tapped M3 Or you maybe able to just screw an M3x10 screw straight into them if you have no M3 tap.
I used the original hole in the middle of the top bar for my left mount and drilled a new one to the right, but you can move and mix and match how it suits you.
I hope this is helpful for some.