Measure from the window (rolled down) to the steel of the door where the speaker mounts. Take that number then subtract the speakers top mount depth. If that number is positive then you don't have to have a speaker ring, if the number is negative then that is the size speaker ring you need; add a little for the curve of the window and to have some wiggle room. You can make the spacer up to 5/8", which I think is what most 3/4" plywood actually measures.