Just bought an 07 LS this past friday, and for my first mod I put on a stubby antenna I had lying around. This antenna is from a honda element. I had it on a Scion tC I used to have before I sold it. It screwed into the Scion perfectly, but that is not the case with the Cobalt. It required extensive modification, but since I had it already I figured what the hell.
Basically the honda element/S2000 antenna itself is the perfect size and shape for the cobalt, but it is the opposite gender. The cobalt antenna is female and the honda antenna is male.
I ended up having to drill out the antenna and remove the small metal block inside. This left a cavity in antenna.
I then cut off the bottom part (about 1/2") of the stock cobalt antenna. Then I sealed off the end I cut so that the peice is only open on one end (where it will thread onto the base on the car.
I filled the cavity of the honda element antenna with 5 minute epoxy (1500psi strength) and inserted the 1/2" threaded cobalt antenna peice into the cavity. You'll see in pictures.
The antenna is nice because it is a flexible rubber and won't break easily. It fits very well to the base, there is just a slight gap between the antenna and the base. It was the same with my tC as well. I think it looks great.

http://www.putfile.com/pic.php?img=3487126

http://www.putfile.com/pic.php?img=3487127

http://www.putfile.com/pic.php?img=3487125

 

Nice job, looks great.

 

Looks nice and clean, good work!

 

looks good. Nice work there

 

nice