Following on from my 101 uses for a dead mouse,if you wanrt a cheap and effective key matrix/randon input decoder you can do a lot worse than the keyboard controller found in that grumby and worn out keyboard that you have in your junk box.
The inputs are nicely debounced with roll-over and repeat.you need to work out how the inputs need to be arranged to simulate a key press but apart from that it works pretty well.