Not to be ornery, but I think the breeder box is a BAD idea in this case. let's think about this -
Let's say (just for the sake of my arguement) that she drops tonight, after you leave. She has all the fry in the box, they fall down into the trap, and everything seems cool. Two problems here. A breeder box is stressful on your female. More stressful than an isolation tank or a breeding net, in most cases. Having her there for a long time (three days) may cause a great deal of stress, which quite often leads to an immunity reduction, and consequently disease.
Also, she's going to get hungry after she gives birth. Very hungry - not right away, but soon. She will be in the box, able to see but not get to the fry, and may injure herself attempting to reach them and eat them.
Just my $0.02!