Pax mid-unit cover panel help

The Pax mid-unit ships with a thin panel with four holes meant to cover the alcove created by the pax and the surrounding full-size units, as it's half depth (so, it forms an empty square in the back). The idea is to prevent stuff from falling back there.

However, the panel seems to be too wide and there are no pre-drilled holes in any of the shelving units to insert the plastic screws. The instructions seem to show the piece sliding on somehow, but this makes zero sense with my setup. Anyone have a clue how to install this thing?

https://preview.redd.it/xn2r84r9d95g1.jpg?width=3072&format=pjpg&auto=webp&s=3655fef6e9ed27a449fe74a2b43e990be0414a34

https://preview.redd.it/rq2jtkbad95g1.jpg?width=3072&format=pjpg&auto=webp&s=a23a7f0312f43275f8d4a05327223d087ab85cca

Page 27 (https://www.ikea.com/us/en/assembly\_instructions/pax-add-on-mid-unit-oak-effect\_\_AA-2594434-3-100.pdf)