Check this out, I modded my PSP by putting a SNES D-pad in it. I used a razor blade to cut the center piece out of the case. I first tested it on my original faceplate before cutting my "Smooth as Silk" PSP face plate. I also used the razor blade to cut down the D-pad. Good thing it is rubberized plastic, which made it easy. Once it fit I curved the base of the D-pad so that the diagonals work better. I also filled in the cracks in the bottom of the D-pad with polymer clay and scotched taped over the clay. Then I tweaked the pad with layers of scotch tape on the directions to get it fine tuned. My PSP went from having the worst D-pad ever to being the probably the best ever. Maybe as good as those awesome Saturn D-pads.
Anyway check out the pictures -- Shoryuken!