I don't want to be a pain in your side, but I'm going to point out something as a result of my own learning curve with pups.
I can see that you have put the cardboard on the outside of your cage, likely so that it is harder to reach and therefore will not be chewed through so easily. What I can also see is that there is some space between the cardboard and the bars. There is enough space there for a pup to climb through, and the pup will either 1. get stuck and into difficulty as it is potentially pressed between the cardboard and the bars, OR 2. it will be able to maneuver enough that it can squeeze out of that area and find itself out in the wide new world. You may need to reconsider your use of the cardboard.
I've got an appointment that I need to be heading for, or I would spend some time thinking of alternate suggestions for you. Maybe others will have some ideas?
1 Bichon/Shi-t-zu Ollie (b'gollie), 1 Standard poodle, Indie
Hi. We put the cardboard on the outside because when the girls are out they have a couple of times climbed up the back of the cage and from there onto my son's bed. As you can imagine it is not easy getting them down from there. The card is actually flush against the bars and wired tight to them in many places. It is in 2 pieces each cut to fit exactly into the frame of the of back panel. I will finish the wire today and last night while trying to sleep I came up with a way to move the shelves around to eliminate the one remaining drop. Yesterday Maisie weighed in at 330g but she is still very active.
THe new shelf upstairs has been extended to reach the front, so no big drop. Then the shelf which was above it is now below it. I think (hope!) it is now baby proof. The green bed at the back right corner is where they have been making a nest. I was mean and cleaned it ou today. I thought it might be the last chance for a while. Have given them back all the dirty fleeces but a new roll of toilet paper.