FrankieShakes said:hkala,
I, personally, use RadTech's ScreenSaverz (http://www.radtech.us/Products/NotebookScreensavrz.aspx).
Have a look... They're well worth the price!
My vote goes to this one. Thin (because you don't need some super thick thing) and very effective. It's also easy to roll up and tuck right in front of the PB so when you're working on it, it's not tossed in a bag---which would cause you to have to search for it when you're done with your PB.