I just built a rotary controller to be able to play Tempest 2000 and Kobayashi Maru in rotary mode (much more fun !). Since I only own one controller and didn't want to spend too much money, I looked for a way to add the knob without making the D-pad unusable.
I've written a mini-tutorial for those who'd like to do the same
Works well, and it's pretty cheap (the total cost was 5€, or about $6.50)