I don't think this is as much of a constraint as you'd expect. Imagine you manage to pull 1 psi over a 10x10" patch of box. That's 100lb of lifting force. If you're worried about whether the cardboard can handle it spread the area and lower the pressure more. I don't think 1lb per square inch of cardboard would be an issue and it looks like they have more surface area for suction than that.