stevey_frac,
I've been thinking over rear skirts and partial fronts since last summer.
And, I think the best first attempt to mount a skirt would be the way that MetroMPG did in his Skirt Thread:
http://ecomodder.com/forum/showthrea...html#post37306
Second picture in, shows his attachment method, using screws and flattened copper piping to provide tabs to mount skirts on.
I don't own a Cobalt, so I can't say where the screws are that hold on your plastic inner fender. On my Jetta on the back wheels there are three mounting screws on the top along the outside edge. Likely it's the same sort of setup for your car too.
What I'd suggest is: Make copper pipe "Brackets" that allow you to use your inner fender screws to hold the copper pipe brackets.
A drilled hole in each end of the bracket will allow you to mount on the existing fender screws, and a screw or other means of attachment to your skirt.
If nothing else, these can be your first-generation / prototype brackets ... you could use aluminum or steel bar stock to make something sturdier, if you find you need to.
Paper/Card/Cardboard strips and a pen or marker might be useful to template out where your bends and holes on each bracket need to go.
The nice thing is, even if you need a different size/shape of bracket for each mounting point ... there's an identical one on the opposite wheel well, after you've figured those out.
You can only plan so much until you have to dive on in... get to 'er!
Oh, and take pictures, if you can! ... MetroMPG was grumbling about his poor pictures in the thread I pointed out. Yet more pictures will get more people to take the plunge after they see how easy it can be.