Answer: It sounds to me as though what you want to accomplish will have a finished look much like an espalier. If this is the case, you'll be pruning and training each plant to a single stem with just a few branches radiating out. Since you'll be pruning in this fashion, you'll probably want to plant your boxwoods on one-foot or two-foot centers (depending upon how large or small the squares will be). Best wishes with your design!
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