I converted the mesh file to a STEP file, tightened up the tolerances a bit (the fit is a quite snug now), and added a lip for a better closing. Also I changed the Magnet hole size to fit the magnets I had lying around. (5x2 and 6x3mm). I also had to drill out the holes with a metal bit, because the holes were a bit too tight. 

Because of the lip I had to print the back part on it's tip (instead of the rim).