Over the years we've used commercially made plastic covers to protect our charts from moisture when using them out in our mostly exposed pilot house. Although the chart books fit into them, our portable nav table doesn't, so I've recently made my own.   I used inexpensive roll clear vinyl window material, but the more expensive Stratoglass would work or even heavy (1 or 2 mil) plastic sheeting from a hardware store or home improvement store could probably work. Cut it large enough to comfortably fit the chart book and our portable nav table (just a piece of plywood cut slightly larger than the chart book) folded over at the top, sides stitched together and a flap at the bottom that can be folded under. I suppose the side seams could be glued or simply taped, instead of stitched.  Not totally waterproof, but will keep most water out, but leaks do occur through the stitch holes, squeezing on some type of sealant would more than likely solve that problem.   This cover could be useful for others in open cockpits or an uncovered flying bridge (in some places, also known as a fly bridge and for good reason). One other thing that we made up and has proved useful are a couple of writing pads that are reuseable and can be written on when wet.   These pads are simply small pieces of plywood covered on both sides with white formica. I guess you could use the formica by itself, but they'd probably not hold up well to hardy use. We've varnished both our portable nav table and the writing pads to keep moisture out of the endgrain.   Haven't yet had the cumption to try reuseable, washable plastic toilet paper, yet.   Rudy Briney Bug- Panama City, Fl