Using an Arduino as an input device
I have successfully reprogrammed the UNO's R3 8U2 to support keyboard emulation, as to just this. Making dedicated user inputs for kiosks to Computers. I recommend NicoHood's GitHub as it allows simultaneous Arduino Bootloader and Keyboard functionality, making it easy to reprogram the Arduino.

This along with using Capacitive Touch Sensor connected to Copper tap under clear acrylic protecting printed graphics of buttons is very quick and easy to make. Very little machining.

Example of such code is found at artemis-firmwares it was initially created with a different 8U2 method. Where next time we will use NicoHood's.

Note the stock keyboard send key command for the Arduino is limited. Where the above methods allows the code to send complete virtual keys supporting full emulation.