I haven't had all that much time to work on the Gameduino between holidays, visiting family and a business trip, but I have been meaning to put together a simple "button box" for the Gameduino, basically consisting of just four buttons (L/R/thrust/fire). I suppose I could have included hyperspace too, but 4 buttons was sufficient for Space Invaders, so...
Here's the thing: I'm not really a wood worker, so the basic idea was to find a prefab wooden box, and then just bore some holes for some switches, and then make a small wiring block below that I could use to plug into the Gameduino. I got the switches from Surplus Gizmos last time I was in Hillsboro, OR visiting family. Seemed simple enough: the AVR in the Arduino has built-pull up switches, so all you really have to do is wire the switches up to ground and the digital inputs, and you are good to go.
So, that's what I did!
Addendum: If you've no idea on how to wire switches up to an Arduino, you might find this page to be helpful. In particular, you need to make sure you enable the pullup resistors on the Atmel by writing a HIGH value to the INPUT port. That enables the pull ups, and makes button interfacing as simple as possible.