I have the same question, so I thought I'd just recycle the thread:-)
I have a unique cab design, (space harrier) So it would be a pain to
extend the panel past the sides and still have it look "right"..
I was hoping to get away with using a couple of control sets I got off
ebay (Wico ball top leaf joys, and 4 leaf buttons mounted in a stainless
steel panel that would mount in rectangular cutouts in the wood CP.)

the buttons and joys are removed in this pic, just sitting there upside
down to give an idea of placement and a copy/paste in PSP of each
red ball top to represent the joys..the two black buttons at the top
would be p1/p2 start, the three reds at the top would be mouse
buttons, and missile command (and other trackball games ) fire buttons..
I only really need four player buttons each (kids like tekken:-), but
I could, if you guys think it's a better idea, cut the button mount sides
off the stainless mounting plates, and just use the joystick mounting
plate half - mounting the buttons seperate from the joys in a more
traditional <closer together> 6 button layout....(I have to get new
buttons that would be deep enough for wood though..)
I don't know,
what do y'all think? :-)