* A spinner CP with 3 buttons (Tempest, Omega Race, Blasteroids). We'd have to use the TurboTwist 2 due to space, which is fine. I hope its not too deep.
* A classic game CP with
[2 buttons - 4way joystick - 2 buttons
]. Ambidextrous, but most importantly the joystick is in the middle. Maybe even do three buttons per side, but form a triangle due to room. There are some games which use a joystick + 3 buttons.
* Williams/Defender panel. Maybe good for Asteroids too if there is room.
You'll get your best ideas from people with Cocktail cabinets. They have to be more creative.
Edit: Let me add that another reason I think these would sell well is
because of the acrylic and uses a thin CP with a metal mount for the joystick. Most of us have no problem building cabinets with wood. Few of us have access to a metal shop. For me, this opens up the possibility to use Sanwa or Seimitsu joysticks with regular shaft lengths, or something like a 4-way Pacman stick which can be difficult on a wood cabinet.