I vote not to use box fans at all.

As a small child, my parents put a box fan in front of my door for air at night. The fan tipped over, facing the carpet and the motor caught on fire blocking the door.

Luckily, the smoke alarm went off we only had one - and my sister ran down the house to my parents room and told them I was on fire. My dad was able to throw a wet blanket over the fire and grab me out of my room.

30 years later...I can't even be in the same room as a box fan and apparently my father never slept naked again.