This is a case I finally made for the FTDI USB/serial converter by Sparkfun at https://www.sparkfun.com/products/9716 (useful for arduinos pro mini, eg.)

It will prevent nasty shortcuts and it is small but sturdy enough that I could probably walk on it. The dimensions are tight but should be all right, else you can tweak the openscad source file. What was important to me is that it was flush on the flat serial connector, else it would be conflicting with angled connectors.

There are two small holes to see the RX/TX lights, and a tight "door" tab to be sled in place tightly. It protrudes slightly on the side so it can be removed by pulling it with a ruler or so.