Are you really going to be in so much rain that treated canvas won't handle it? It seems like you would not be able to shoot in such conditions anyway, so why not just a highly water resistant material? I doubt that pure rubber would be that comfortable to work out of, not to mention not breathable, so you might build up condensation. Some of the best stuff is probably the Nytex bags by Billingham, but you can probably also take a Domke or Safrotto and use some canvas waterproofing junk from a camping store. If you need a truly waterproof case (as in submersible), then Pelican cases are probably the way to go.