WOW now I gotta do my rear pan on my xB - tape is not permanent and will always let go - a really heavy sticky shipping tape may work better that duct tape until it gets wet a few times. Better to tuck the rear edge above the bumper if you can then secure the leading edge with bolts and nuts.
Just a warning to you guys. Tape and the metal won't last long. My tape is holding on for dear life and the metal is bending. I'm going to try to fix it today. There is just too much force on the metal from freeway speeds.
Yeah you have to figure the area that is exposed to the air flow - the bigger it is the more forces are on it. That's why I recommend fastening the leading edge well so that back end can blow out adn still be connected to the chassis without the leading edge scooping in more air. I am thinking some glass and epoxy over mine to strengthen the foam board and giving it a lot of open space on the sides to allow air to flow across it and cool the muffler. Time for a "before" shot while the camera is all charged up and the sun is shining. It can be so embarrasing when a chunk of your car falls off on the highway!