A simple Rack and Pinion motor mount with a rail in the vertical or horizontal position. Customize the rail's groove as you need it. The rack used was 4mm x 10mm. Great for stepper motors (the one used was a Jameco 161373).
To mount a motor to the plate, use some zip-ties or velcro to attach.
Rack and Pinion not included. I used the one by rushman: http://www.thingiverse.com/thing:1227374
** Note for remixers** Should probably round out those corners and cut down on excess plastic on the base. The height is probably overkill.
** Update (2/20/2017): ** * Added a horizontal option * Base plate thickness from 2.5mm -> 2.0mm * Rail length changed from 35mm -> 30mm