Yeah, four micros would be too big to fit, wouldn't it? If you are handy with a dremel, you could cut out some button pads from a wrecked controller. Even a basic joypad will have at least six or so. You could arrange them 'standing up' like this (looking down from the top):
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l_l
with the rubber pads facing inwards and the joy obviously pushing on them in the middle. This would be very compact with a trade off of extra depth, but with luck that can be accommodated in the cab without changing any dimensions (",)