aluminum is 30% the weight and 50% the strength of steel (this varies greatly by type, the harder the aluminum the more strength, but the less it's able to be worked/bent/shaped) Thus, if you want the strength of 1/8" steel, you'd get 1/4" plate aluminum, and it'd weigh 60% of steel... if you can get away with 7071 (any 7 series, actually), then you can decrease to 3/16" aluminum for the same strength.
heat - most race cars run full belly pans and have no problem with overheating. Most SCORE and trophy trucks have full underbody armor. Most BMWs and VWs have full belly pans and don't over heat. In addition to the benefit of protection, you'll also get better mpg because the air will flow under your car easier.
Fire extinguishers - if you still use powder, you really need to change now to either liquid or foam. Years ago, I stopped to help a lady whose s-10 blazer was burning with an electrical fire. I used a 10# fire extinguisher and would knock the fire down, wait a bit, knock it down... did so for 7 minutes. The fire department got there in 12 minutes... the car was a total loss because there just wasn't enough powder to keep knocking the fire down until a truck with water could cool the vehicle enough to end re-ignition. In the recent year, there's been at least 2 companies that use the same tech as Range Guard fire systems to put out fires. They work on two principles: 1) the liquid coats the surface with a non-flammable liquid thereby depriving the fire of oxygen, and 2) it cools the area thus reducing even more the chance of re-ignition. Best part of all, it's about 1/2 the size of most people's fire extinguishers and they cost about the same amount.
