I always have a box of 45 gallon contractor trash bags in my garage.  When it comes to canoe trips, I use one of those bags inside my pack.  They're heavy duty and never have torn.

As for the top, I simply twist the top as if I were going to put a rubber band or zip tie around it, curl it over and close the pack.  Pressure keeps it shut.  Everything stays nice and dry.

The rubber band, or even a Bungee Dealee Bob, would be better, though.