The last trade-in package I received for an iPad used a single sheet of plastic with some clever cardboard engineering.
The cardboard comes with the side flaps (to which the ends of the plastic are glued) folded back, stretching the plastic taut.
When the side flaps are pulled to the side and then forward, the plastic loosens enough to slide the iPad under the plastic. (The diagram shows the iPad screen should face the cardboard.)
Then you fold the side flaps back where they started, which pulls the plastic taut again, securing the iPad to the cardboard.
Note that the above description should work for an 11" iPad Pro. I found that a 12.9" iPad Pro required me to perform an additional step to make sure that the side flaps folded back at a different place (subtly marked by a slight perforation).