I needed a microcontroller-controlled RS-232 serial mouse, and had an Arduino Mega 2560 laying around.
Files are in KiCad format, the Arduino shield outline and placement was based on the Eagle file supplied by Arduino and converted by the script (https://github.com/DanChianucci/Eagle2Kicad).
All 8 RS-232 pins are accounted for, not just RX and TX.
You can see and order a board from OSHpark here: (https://oshpark.com/shared_projects/Vs4zWf41)