Um ...you just blocked off the air that goes to the radiator. That big black thing is a spoiler that directs air upwards into the radiator through that hole that you covered. It serves double duty by steering the airflow away from the underside of the car.
I noticed that you still have the main area of the grill open. If you covered that up there would be no air at all getting to the radiator at all now and you would burn up.
GM did a really good job smoothing out the underside of their cars ( my mom has one, and I looked under it )
The way that you have it now you could actually take that spoiler off and see an increase in FE due to less frontal area. You would have to fix up the area under the bumper though, because it is the one area that Saturn screwed up on ( It is open like a parachute. )
I'm not sure which would help your MPG more - taking the spoiler off and having the grill open or blocking the grill and leaving the spoiler on. It would depend on if you feel like smooting out the cars underbelly , or just blocking off the grill ( That would be the easy thing to do. )
If you unblocked the area above the spoiler, you could completely seal the entire front end and not have any cooling problems. ( Look at the early SC1 front ends. They didn't even have a grill opening .( The ones with the flip up headlights ) Just make sure and unplug that area above the spoiler if you seal the front end openings.
Nice car. I like the body style -it was a cross between a station wagon and a sports coupe ( SC2 front end ) ... but then I like hatchback cars anyways