building box 'play areas' for your chin to hop through and hide in during playtime is fine. you do want to keep an eye on them to ensure that they are not actually ingesting any cardboard. chewing a little of it is fine though. don't keep cardboard items in the cage though, because that does raise the risk of them actually eating it and getting an impaction.
i make cardboard play things all the time for my boys, with different boxes. some have print on them, some are just plain brown cardboard. just be sure there are no staples or tape anywhere on the cardboard, and supervise the whole time.