I use a bottle cover, its not really to protect the bottle its to stop it slipping when I hold it in my shoulder. Redz do quite good covers if you are thinking of getting one.
Why not have a look at the Lok-Soks? They look great, protect your tank where it can hurt the most, and really help shouldering the gun. Your tank will grip much more. I never was too fond of tank covers, but they do make a difference IMHO.