Hey Seablade, I am surprised that you say you haven't had spill problems with guitar mics. We obviously have different definitions of a loud guitarist. When there I have 4 vocal mics I find spill a big issue, when the player steps out of being in front of the mic. Recently taken to using Beta 57's (Tight pickup pattern)for this reason. Still this whole box idea seems a little crazy.
You misunderstood. I don't have problems of spill into the GUITAR mics. Spill of guitars into vocal mics is almost always an issue, but generally lately I have worked with guitarists that are quite talented and know when to control their own levels so I haven't had to worry much except when I do the odd freelance one off gig for new bands.