i know thats an old thread, anyway here are some pics of my ghetto pop'n music controller. Just built it yesterday, took me 3-4 hours (im slow with my hands).
i bought 6 tennis balls (1 extra) for 6 euros (8.6$)
and a cheap keyboard for 10 euros (14.4$)
The final product without paint looks like this:
Without the tennis balls:
One sanwa button
ghetto style:

strange odor+rasp feeling+invisible tape and cardboard=win*3
got fun with the tennis balls (harder to kill than they look)
Oh, and i disabled all the keys except for the mapped ones, with a pierced paper between the circuits sheets:
my keys map:
4 9 Back prior
Z B . left numpad3
You need vjoy (keyboard to joypad input software) to assign the keys to buttons, as LR2 doesnt recognize the left key and the numpad.
No need for vjoy in osu! taiko mod (the ghetto minicon can be used as a tatacon, its sturdy enough)
voila