When we built our welping box as it need to be fairly high at the sides so that the pups couldnt climb out or fall over the top of mum, I was concerned that mum would not be able to get out of the box herself to go to the toilet etc, so we inserted a fold down gate into one corner of the box, this could be lowered for mum to get out and then put back into place after she had returned. the rails need to be high enough for pups to crawl under comfortably but not high enough for the mum to lean back and get under it herself, the rails are so that the pup, should it get trapped by the mum could wriggle free and under the rails to safety.
Mo