You should probably wait until she, "squares off", before putting her in the breeder box. Her belly will no longer look rounded at the bottom, but more flat...From experience, these breeder boxes cause a lot of stress, but are good for catching all the fry and keeping them from being eaten or getting sucked into your filtration system. If you are able to, a better option would be just to keep her in a heavily planted tank or aquarium where babies can hide.
Good luck!