Garbage bags are good and they are cheap. They will last forever....just make sure that you use the Heavy Duty ones. Another good bag, for the inside of the valise, is the 'Shitter Bags' from the Thunder Boxes; if you don't mind an Orange bag inside you valise or ruck. Sometimes you can find a clear plastic bag, that some of your kit may have been issued in, that is of a heavy grade plastic, and use it. By using these means you can throw your ruck and valise into the wash and clean it, and then not have to worry about some expensive Silicone spray to water proof it.