You could use a rotary switch to set up send returns for each circuit and you could use a single volume control by using each fuzz's "volume 3" wires as the return (omitting their individual volume controls) and just using one volume pot post rotary switch going to the output socket. Hope I've explained it clearly enough to make sense...!?
That, or look up what sizes Accutronics makes its spring tanks and build a rack mounted spring reverb? :)