I have an "Ortlieb" water resistant bag. It's a hard material and then a rubber covering comes over the zippered area. It is clipped to the underside of my seat and a small velcro around the seat tube. I think its a great idea. Holds my tube, CO2 cartridge and tire pump, levers, multi tool, gum, and eye drops. I bought it so my cell phone wouldn't get wet. I have to carry that (on potential rainny days in a plastic bag) in my jersey along with snacks as there is no more room left in this bag. I am not sure what size I bought, but i wish I had taken all the goods with me when I went saddle bag shopping. Besides, this was the only one on the shelf, he said they had sold out. Good luck in your shopping experience.
PS. My saddle bag on bike #1, holds everything but the kitchen sink but isn't waterproof.