I have a polycarbonate cover, it's hard plastic piece, but be warned it will produce some glare (not terrible amount, similar to looking at a laptop screen) but it hovers well over the screen, so you could tap it quite furiously without damaging it.
Don't use those LCD screen films. They don't do much, and don't protect the screen from pressure, and still add significant glare. Also, some poorly made ones can damage the screen coating and pull it loose from the screen damaging it permanently (though this is rare)
This is the product:
CLEAR mCoverŽ Polycarbonate Kindle DX Hard Shell Cover Case for 9.7-inch Amazon Kindle DX
http://www.amazon.com/mCover%C2%AE-P...7412217&sr=8-2