Made by twin parents. We left this sign on our door until the kids were 4 years old.
Trust us as soon as you get that 20 minutes to close your eyes between feeding and diaper changes X2.......ding dong. Hi, would you like to..........blah blah blah blah.......slam the door. Its like they have some sort of smart radar for when not to ring or knock.
This sign is not guaranteed against the dumbest solicitors but our interuptions were down to about 1 every few months and yours should too.
Its almost worth the interruption when you open the door, look at them, look at the sign, look back at them and close the door.