Channel catfish prefer to spawn in water two to four feet deep. Be sure the box is only open on one end and there's a lip at the bottom of the opening so the fry can't roll out while still in the sac phase. Aim the open end to the south (An old timer catfish farmer taught me that factoid 30+ years ago...seems it has something to do with photoperiod).
But the biggest question is...why do you want your catfish to spawn? As inexpensive as they are to purchase, you stand to lose control of their numbers if you have a successful spawn.

Teach a man to grow fish...
He can teach to catch fish...