do you want to be able to just turn them off or also adjust their RPM's?

i think rheobus is the right way to do it but the rheostats takes up more space than a simple switch

how about this...
have the fans near the front hooked up to a rheobus
you can try to make it yourself
try to fit 5 rheostats onto one 5.25" plate
or try the zalman one which has 4 rheobus and 2 switches

then make some switches on a pci backplate or more rheostats on a floppy plate

up to you