I tried a few ways to accomplish this...i thought about doing it the way you have it, but it bugs the crap out of me to see the title of the bags through the background of the others...
this was my solution....since the theme i'm working on uses only a 1px border around windows it doesn't look too bad.
here's a pic:
http://www.the-mutiny.org/Buroku/bossyUI2.jpg
if you don't like the border then try just making a solid black background w/ a solid black border.
just a thought =D