Thanks for the input guys. I've seen the ipacs before but was hoping to find a cheaper route. Then I've read about keyboard hacking and soldering. The uhid nano seems to be what I'm looking for. Need to get some more information though. I would run a happ 4/8 way joystick, 6 buttons, 1 player start button. Separately run would be an audio on/off switch with analog audio dial, marquee light on/off switch.
This project being a mini cab wouldn't need a coin slot. Also I do plan on running MAME on it but it won't be the only things. Plan on running NES, Master System, Atari, Commadore, TG-16, Genesis, and SNES. hence the 6 buttons for the last 2 systems.
I made measurements of the donor monitor last night (without it's shell) and looked at Tiny Arcade UK for inspiration on the cabs final design. The monitor is a 4:3 so I may have some smaller images when MAME'ing the "tall" arcade games such as Donkey Kong or burgertime but it's a concession I can live with