I would seriously wait till you have more money and get a butyl mat. Yeah it may be ok but you do not want to try to take the stuff off after you but it on. Do it right once so you dont have to do it again. Id do one layer on the outer door skin for the speaker, one layer on the inside of the door where the speaker mounts to. You can do more layers but each additional layer will have diminishing returns. Its how much you want to use there. Knock on different areas and put mat where ever it sounds hollow. I noticed the area on the back qtr pannel is also very thin and would probalby benifit from mat more then other areas. Also ensolite foam will also help with road noise and its relativly cheap.