The temperature is a good guideline but not a sure thing. I have taken females temperatures and would get a normal reading and within a couple of hours they would have puppies. Sometimes the temperature will go up and down for a week. I have found that with my pregnant bitches that often I would get a low reading in the morning and by noon the temperature is normal. The variation sometimes can be contributed to low blood sugar. In the mornings if they have not eaten their blood sugar can be low and cause a lower temperature reading. In most cases if the temperature drops and stays down then they will usually have pups within 12 to 24 hours.
As for the whelping box. Most females will not use the whelping box until it is time for delivery. Even then you will probably have to sit right with her to make her stay in the whelping box. Most females would rather have them under the couch or bed.
Best wishes for a safe whelping.