Okay so I've finished the design and printed out some test clips.
The prints came out okay but I noticed that the ABS material I used is not as flexible as the original. With the dimensions the same as the original part I suspect that it may suffer the same fate and crack in use. The fix is a simple one and that is to remove a little material from the clip so that less force is needed to secure the sun visor. A better alternative would be to print them in one of the flexible filaments now available but this is not possible on my machine as it has a bowden style extruder.
I've posted both the models and the generated STL files up over on Fabribles. You can also view a 3D model of them there too. Feel free to download and modify as required. I designed these in Austodesk's 123D Design. This is a free application. They offer a downloadable / installable version or an in browser version. It is also available for your iPad. I have included the source files so that you can view and edit the model as necessary.