In this remix, I added depth to the back interior to better clear the microphone on the back of the transmitter board. I also extended the lips and recess where the back and front meet so it snaps together better.

I added two additional back pieces each with tabs for screwing or otherwise mounting the back to a surface. One has tabs on the ends and the other has tabs on the top/bottom so you can choose your mounting solution.

The case fits the following FM transmitter available through many sellers on eBay:
http://www.ebay.com/itm/282008791252