The keyboard has four square keys made from conductive fabric glued to cardboard. Each key has a small section of copper tape with conductive adhesive. A short wire is soldered to the copper tape. The other end of the wire is soldered to a capactive touch sensor on a PJRC Teensy LC board. The board has 11 touch sensors so 6K and 8K games can be supported plus Enter, Escape, etc.
This works fine as a four key beat pad but really is a mock-up for a dance pad. Scale up to 1 x 1 foot conductive fabric squares. Go hard by gluing the fabric to plastic or wood. Or go soft by sewing the squares to an old rug. This should work for rhythm games that accept USB keyboard input such StepMania, DJMax, Musynx, and Deemo.
This is a mock-up for a 6K cardboard piano. I thought about using it with Deemo:Reborn but a MIDI keyboard is better.
The Arduino sketch scans the touch sensors. When touch is detected, the sketch sends USB key presses for keyboard Left Arrow, Down Arrow, Up Arrow, and Right Arrow. As far as the computer knows, it is talking to a USB keyboard so no drivers are required. The keys can easily be changed to WASD or DFJK.
The copper tape and conductive fabric came from Amazon. The Teensy LC is from pjrc.com.