Still need to seal up all kind of potential leakage points. What you gonna burn that much charcoal in and where are you gonna place it so as not to start a fire? There's a procedure for getting the charcoal burning correctly before bringing the charcoal into the car, so you need to familiarize yourself with it prior to executing your plan. Without doing the math (.135 kg/m³ of charcoal needed for a given space), an 18 lb bag is *probably* more than enough. Still, it needs to be done correctly or you can have disastrous results. Do yourself a favor and read up on everything you can about the CO method.