Alternate trunk lid
 

I was thinking of creating a fiberglass trunk lid for my Cavalier that instead of having a flat top, then curving in and down (probably creating a low pressure zone), would in the center, follow the same downward angle as the rear window until it meets the bumper. The left and right taillights would still have the original slope since they're part of the body, but would slope down sharply towards the middle. I'm thinking that a cross section of the middle rear of the car would end up looking a lot like a Porsche 911. In effect it would be a semi boattail, but instead of adding material to the car, I would take it away. I would lose some cargo area but it would be no more difficult to park and few pounds lighter. What do you guys think?

I would do some tuft testing on the car as it is now and then remove the trunk lid, make a cardboard mockup of your idea, and then do the tuft testing over again.
If you have a small webcam you could mount it on the roof of the car ( out of the way of the airflow of course ) and then just record it to your laptop which you would have in the car at the time.
Post you results here, since we all love to see tuft testing. ;)

You need to post your vehicle(s) in the garage so we know what year/body style you're talking about ;)

http://ecomodder.com/forum/emgarage.php?do=add

Whether it's going to work or not will depend on whether there's currently attached flow down the rear glass onto the trunk lid. If there's not, and an entrained bubble is contained at the base of the glass, changing the current trunk shape might actually make the situation worse.

Tuft testing is a good idea.

Aerodynamics is a funny science. What the eye tells you is slippery may not be. Check out this article on the Porshe 993, a 911 variant:

http://autospeed.com/cms/A_108675/article.html

Going back to the Turbo 911s of the '70s you see a big-assed spoiler on the back. It's not there for looks and it's about the size of a trunk deck. Hmmm...

Keep reading further down the article about the Beetle (like the one I have) and you'll see the shape can cause a surprising amount of lift and drag. I'm looking into a spoiler to put back there now.

What body style is it? Sedan, coupe, hatchback?

It's a coupe.

Gee, the coupe doesn't look bad to me.
http://carblueprints.narod.ru/images...coupe-2004.gif

You'd get far more bang for your buck starting at the front. Block the inlets. Lower the car. Partial belly pan. Wheel skirts would help too. Those things are easier than trying to make a boattail. Do a boattail as a last resort, IMHO.

Looking at pics of the coupe, I'd say you probably don't have attached flow all the way down the window (angle at the base is too steep), so modifying the decklid as you proposed would hurt, not help.

I hope not like the 70's 911, its just as bad as my 80's saab 900, and the small factory spoiler did nothing, I added a Saab Whale tail spoiler and It made a difference

OK I guess I'll leave it alone. Aerodyamics is a huge blind spot in my education, so I'll trust you guys on this one.