Around exhaust, and the back of the engine bay. If you were to completely cover the bottom of the engine bay, you would effectively seal the pimary exit for the airflow entering the engine bay. that... is not so good as then it won't enter the engine bay in the first place.
I covered the entire underside of the engine. I left only vents under the exhaust. I just fixed two access panels that allow easy entry to the oil filter and trans. Eventually I may put some hinges to make it easier to get to. It takes me about 10 minutes longer to change the oil because of the pan. It rarely runs hotter than 200f. In the hottest part of the year it ran 208f on the interstate at 80mph with a/c. The system was very taxed. But if I'd slowed to 70mph it would have been fine. Also I find that when I draft temps run a little higher. The cat is 2" inches from my pan and it shows no signs of heat damage. The muffler itself got very hot and cooked some paint off but thats it. Soon I will cover all the vents, seal the front wheel wells and rearrange the engine bay exhaust under the car.
02 Saturn SL
for pics click the link below