Based on thing http://www.thingiverse.com/thing:83102 which is based on thinghttp://www.thingiverse.com/thing:39519 
I wanted a fully parametric OpenSCAD version. It's simple and the STL I uploaded is for a 26x13x3mm magnet. I printed two and they hold well. If you try to print the wall thickness under the magnet, it'll bulge too much and cause the bottom to rock (not good). 
Having just used this, I would recommend that the grooves be deeper and thus the thickness be greater too.