Like the original model, print both of these (I used PETG) and line them up with the cutout for the parcel cover's rear perch. Line them up, drill a hole suitable for an M3 screw, insert M3 screws and nuts and tighten. Works just like the original clips.
I used M3-0.5x20mm but those were a bit long. M3x14mm would probably work.
The author remixed this model.
The slots were pulled together 1.2mm total (0.6mm each side) which fit my 2017 Ford Focus Electric parcel cover perfectly (original model flexed the back out too much).