I borrowed from Sparkfun's design for their USB-to-UART FTDI FT232RL breakout board as well as the examples on the FT232RL datasheet. Then, I added a SN75176B (a MAX485 clone), configured for data output mode and two RJ-45 jacks. A 120Ω or 60Ω resistor is needed to terminate the lines (depending on whether you use 1 or 2).
There's a few status LEDs showing power, TX, and RX status too.
These files contain the full plans for the board. I use Kicad for schematic capture and board design.
I found all the parts I need on Digikey, but it's possible to get some of them cheaper on Avnet.