I created my "Arduino Mega 2560 r3 bracket for Extrusions I-Type 5 slot." at http://www.thingiverse.com/thing:989845 and originalle started designing this vertical version. I went to horizontal version early in the designstage, but finished this one, as I bet some people would like it oriented like this. Added Photos from my Horizontal Mount to show how the plastic-zipties Works - you do have to reorient them a tad though, but I'm sure you can figure it out. There is no room to use actual screws for those two holes. I printed 0.2 layer height and 20% infill. Plenty strong. You can print either the 1-piece model, which doesn't have the connecting braces in the middle, or print the 2-pieces bracket. You need 2 plastic-zippers for the two top holes (see photos, on how to orient them) and 2 M3 8mm screws. I used hexagon socket screws. BOM 2x M3 8mm hexagon socket screws. 1x M3 8mm flat headed screw (for the 2-piece version) 2x plastic zippers. To fasten bracket: 2x M3 8-10mm screws.