I picked up an R-Type cab for Mameing & I also have a Donkey Kong cab which is pretty close (panel, Marquee, & monitor glass all can swap out).
You have basically about 20" x 7" of realestate in which to mount your controls.
The panel; itself is a bit larger but you must account for locking clips & the base edges.
I laid this out on paper this morning & if you tried realy hard you can fit two sticks with 6 buttons each as well as a 2.25" trackball in that 20".
I'm not saying that it'll be comfortable, but if that's what you want, it is do-able.
Goodluck
Z
