My sedan has the same bumper gap for years. I gave up trying to adjust since it kept popping out. Your idea has my hope rekindled. Also, I subbed to this thread.While waiting for my front bumper, I decided to fix my rear bumper fitment. I didnt get a chance to take a picture on how I did it but I took off the bumper and applied tension on both sides by using a masking tape on one end to the other to push both sides in. let it seat under the sun four couple of hours and boom!