Most of cover held on with one time use pop fasteners which loosen when worked more than once. The underlayment under cover at least on the earlier cars is a form-fitted piece of styrofoam and you will never pay more for a piece of it than that piece if you have to have it. Sometimes they break up but if you recover the pieces before they blow/fall out to be lost you can glue them back together to make a good tight cover again. BTDT. The light metal 'bumper' (a joke) that supports the foam can mash in too but pretty easy to rework them to make a tight bumper cover too unless really mangled up. While doing that work I pretty much replace all the pop fasteners with positive screw down bolts or other but you have to size them out for yourself, there is NO list and I'm not going there.
My cost maybe up to $20 for the various fasteners used there, I did not paint as car was old and only cracked paint, it's still there. The cover though is in good shape and tight, in fact better than OEM. After TWO hits from the rear.
That on like '02 model, the later one will use many of the same techniques with varying details. All unibody/outer panels essentially the same.
FYI, back in the working life day of the '02 a simple unpainted bumper cover alone went for like $350-$500.