Most of the cars I've owned have only two bolts at the top, leaving the bottom of the plate to vibrate against the car. I know it's kinda janky, but I applied a plate-wide strip of 3M double-sided foam tape across the bottom of the plate, tightened the top bolts, and pressed the bottom of the plate against the car. Use a thin black metal frame, and the neat & tidy black plastic caps on the top bolts, and done. No rattle / vibrate, no exposed bolt heads.