Maybe you've thought about this and rejected it but...
Take a look at Aqua3. They will coat maps with a waterproof and very thin plastic film. Not only does it keep the maps dry and stops them falling apart in the rain but it also makes them more robust even for dry use or if a gust of wind should catch the map. Another big, big plus for the Aqua3 film is that you can write on it with a Sharpie and then wipe the writing off with some IPA, or probably even vodka, allowing you to mark up maps in the field and then remove what you've written.