if ur flying next to the plane that is being targeted and ur in the box as well then there is a chance u will get hit.
dont forget the fuses are proximity type so if the shell misses its intended target by a big enough distance NOT to detonate the shell then it
keeps going till it does reach a target (possable you!!) or comes back down to earth/sea.
if ur in the way of the shell to its intended target u get hit. this can be seen when ur in 5" shooting at cons about 3-4k when the furballing with friendlies, some time ur ack will detonate at the friendly not the con.
There is mixed, misleading information here. The only proximity fuses in game are from 5 inch shells that are fired by a player. Auto puffy ack is not proximity based, it is in fact exactly as the OP described, a "box" around your plane.
Manned 5 inch shells will *detonate* upon proximity to a friendly, but will not cause damage to said friendly.
If you, as a friendly air asset to auto puffy ack, are reasonably close to an enemy air target, it IS possible to be killed by your "own" puffy ack. Again, not by your owned manned guns, but your own auto ack.