How can you prevent a smooth underbody tray from getting exhaust fumes inside?
 

Making a smooth underbody looks easy.

How can you prevent exhaust fumes from getting inside the cabin?
Maybe you should leave an open space at the rear?
Or maybe some vents?

Just make sure the end of the exhaust pipe sticks out of the bellypan I would think would be enough.

That is all I did and I have had no problems with fumes. As long as your exhaust and floor aren't both rusted through you should be fine. Some folks with hotter exhausts than mine (gasoline fueled) leave out a channel of coroplast close to the pipe so the plastic doesn't melt.

Unless there's a leak in the exhaust system, it shouldn't be a problem as long as the tailpipe isn't covered up.

1. Have you looked at any of the smooth under belly panels posted on this forum?

2. Every car sold in america has to have exhaust tips that send the exhaust past the car.

Given those two statements I cant really understand your question.

Here is an example of a rear panel ( on my car.....see links in mt signature)
http://i678.photobucket.com/albums/v...pgq45115-2.jpg

Don't worry about it.