After reading this question, I thought up a scenario that I wasn't sure if it was answered inside there. If you have a normal use of the Alarm spell filling up a 20-foot cube, could you leave a space the size of a 10-foot cube on the ground in the middle to be unaffected by the spell? A reason you might want to do this is if you have a creature captured and bound, and you want to be alerted if he leaves his space. You would cast the Alarm spell on the area around him (within the 20-foot cube), leaving the square the he occupies out, otherwise you would be constantly alerted, as the spell states (emphasis mine):
Until the spell ends, an alarm alerts you whenever a Tiny or larger creature touches or enters the warded area.
Is this hollowing out of the Alarm spell possible?