On mine, hatchbacks, the bumper foam is what holds the cover in position. There are 2 plastic pins that hold the foam in place, and for the life of me I couldn't figure out how to remove them, ended up tearing a hole in the foam.
All I can say is to remove the bumper and see what's back there, or not.
Don't "think outside the box".
There is no box.
Do it correct, or do it twice.