The picture provided is a great whelping box. Kudos on the box.
However, it supports my idea that the bumper may be a complete waste.
As much bedding is there, the bumpers are no longer bumpers, but are merely the wall now. The straw, bedding, builds up and there is not space for the pup to be.

I would accept the bumper as a good idea if there is no bedding at all, but if there is some, which we almost always have, then the bumper to me is a complete waste.

Not being critical, just conversating.