If you mount a speaker that has a 2 7/8" mounting depth directly to the shell of the car, it will just clear the window as it rolls down. If you have a deeper speaker, you can pull it away from the window by mounting it on a 1/2" to 3/4" ring made of wood, plastic, or metal. MDF is a common material used but it tends to absorb water. People have good luck using cheap plastic cutting boards too.