Well a heavy rock can do the trick pretty easily... unless you want to, you know, put it back together.
I don't have access to a controller right now but I do know that the Cube itself requires a gamebit to open so if the controller uses the same screws you can use one of those those. Play Asia and a few other game import sites sell those.
If the screws are the same as a GBA's you need a Tri-Wing.