When I was in college, we would first paint the floor with some thick enamel paint. Let that dry really well. Then get about 3/8 inch thickness of newspaper, wet it with a mixture of water and flour (roughly equal parts) and press it firmly against the floorboard on a dry day. Your 3/8" will then be more like 1/4". Run it from floorboard to inner firewall and across from side to side. Let it dry completely and if you've done it right, it will perfectly follow every indention, bolt head, channel, etc. Once it dries, it will sort of resemble a thick egg carton. Carefully remove it (note: you may have to cut it in the middle with a box cutter...sometimes you can't easily remove the entire piece), lay the back side up and coat it with a couple of coats of polyurethane to give you a vapor barrier and for strength. Let this dry and recoat. Then set it back inside and coat the top side with polyurethane. Now you have a custom mat that will absorb noise and keep your interior more warm in the winter. A friend who had a 49 Plymouth made it so quiet you could hear the car's clock ticking.
