Here is an image of the pan on my truck.It's fiberglass and aluminum.
I've taken no care to accomplish any special shielding or insulating and at around 7-years now,have had no issues with lubes or fuel.
I have leakage at the front suspension.The coooling air can escape to the front wheel wells,and also,there is a full-span gap at the rear axle ahead of the diffuser.
So far,deep water is the only thing which has impacted the truck.I drove slowly through a flooded alleyway and the force of the water was enough to rip the diffuser off because of it's sealed rear bulkhead.
http://www.evworld.com/images/pknox_bellypan.jpg