Neo Geo controller adapter seems easy. Take a working PCE controller (ugly or busted shell preferred), remove the shell and wire in 15 pin connector to corresponding pad on PCE controller. Tuck in a box.
Neo Geo controller has 4 buttons, either 2 can be left unused, 2 wired to rapid fire option while other 2 are non rapid fire, or steal 3 button Avenue design and map Select and Start to other 2 buttons.