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I suggest a corner placement for the installation as this hides the back surfaces from view.
However by adding a second projector the rear surfaces could be utilized with an additional box added to the centre back to balance the arrangement. The installation could then be viewd from 360degrees. Also as the projector is mounted at a high angle they could be approached without casting a shadow
Video and equipment list
1 or 2 XGA projectors standard lens 1.8 throw ratio
1 Mac running Max/Msp 5.1
4 or 5 White Box Card boxes 800mm x450x266 approx (overall size dictated by projector throw and distance from objects and can be scaled appropriately, the greater the distance the larger the objects)
1 pair powered speakers or Amp and 2 speakers with stereo input.