Wintermute

You could use something like b-quiet which is a sound dampening material for use in cars (I've used it commonly in standard passenger vehicles and it works quite well). The primary things to pay attention to are the methods of installation, and the fact that in a passenger car it's meant to be installed on bare sheetmetal underneath carpeting and such. Since it doesn't cure over time (it stays tacky indefinitely) and since it doesn't have an aesthetically pleasing quality, it's best to have it covered up. But it will reduce noise overall as long as your adequately cover the area in it. Remember that you end up with two primary reasons for noise transference throughout a vehicle; vibration and open airways. With vibration, most automotive sound deadening materials are heavy and flexible which provides a vibration dampening effect to the metal they are adhered to. For the open airways, you really need to seal things up and ensure you have a full covering across the surface with the sound deadening material. This will reduce direct noise transference through the air into the passenger compartment of the vehicle.

Hope that helps, I can provide links to some different sound deadening material providers as well as some advice if you need.

--Wintermute