it really depends on what is more important to you..form or functionality.  My cabinet was similar in dimensions to yours when i got it.  i knew that i lacked the carpentry skills, and money to make a stunningly beautiful cabinet like some people have, and no matter what i did, it was going to look like a beat up arcade game that has been used for a while. with that in mind, i tried created control panel prototypes that would fit within the dimensions of the cabinet, but nothing i came up with was comfortable for 2 adults to use at the same time.  so i notched out the cabinet and made a panel box that overhangs on the sides and front slightly.  just a few inches made a huge difference. my panel is 30x12 now and there is plenty of room for everything.  i had 2 players with 6 buttons each, 3" trackball, spinner, 4way stick, and a few admin buttons.  i have since removed the trackball and spinner since i never used, but i am very happy with the way mine turned out.  It isnt nice like some of the cabinets on here, it is very plain, all black, and a little beat up looking...but that is what makes it feel right to me.  I am constantly making little improvements here and there to make it look nicer, but i decided functionality was more important to me and I stand by that decision.  Cutting the cabinet and making a new panel was the best thing i could have done to the cabinet I have.
IF you would rather have something that looks amazing...i'd go with swapable panels.