Body mods come in a wide variety of materials. These all have different strengths and weaknesses. Some are lighter, some are tougher, and some are easier to customize.
Steel may be heavy, but it is also tough and easily repaired. About the only thing it does not hold up well against is road salt. The most common off-the-shelf steel parts used in customizing are imported parts installed as original equipment in other countries, but some companies offer aftermarket steel hoods with a variety of scoops and other shapes.
Steel parts can also be modified. If you want louvers, a local machine shop can punch louvers into your original body panels. More complicated features can be made by welding scoops or other shapes to the original ...