This car phone mount can be customized to accommodate a wide range of phones and cases. It clips onto the vent using the really nice vent clip designed by jasper_pues. The concept was inspired by the gcode developed by diametric.
Update Apr 20, 2015: Unfortunately I had to remove the rounded edges due to a problem with the customerizer.
Use the customizer to put in the dimensions of your phone or case. The sized you put in will be the size of the space on the holder. You should add one or two millimeters to the sizes you measure on your phone to make sure you can slide it in and out easily. The vent clip is printed separately and should be glued into the slot once the prints are done. A couple of drops of superglue works great. The clip prints stronger if you lay it on it's side, if you do you'll need support, otherwise, no support should be needed. You may need to clip off a string or two from the mounting slot depending on how well you printer does bridging. Obviously, your printer will need to be well calibrated to print this the correct size.