I use my net all the time. It does cover the trunk opening but you can push it down (it has elastic bands). Not sure how much it costs - GMGoodwrench.com does not list the 04 parts yet only the 02/03 accessories. It came with my car but I am not sure I would have bought one. Although I do use mine all the time.

I use it to hold those little things that I do not want flying around the trunk. Like right now I have a mini Box-O-Rags in it. The paper towels in a box.

I also have a 2x3x2 plastic bin with lid in the trunk to hold "dirty"/Wet items got it for a few $$ at Wal*Mart.