Just curious....is your mama dog and her offspring the same breed as the newcomer? Will this box you are building be in your house......
I, personally, have never had a dog relatively newborn.....always rescue mutts, oops, now they are called " designer dogs"........nor cats that young either.......I don't want the hassle of housebreaking.......we have not had ant problems with our rescues....all have been great, loyal companions thru the years.........best of luck with your new baby.......