I use a double zip-loc. Not perfect but hey, it works. The white logo and label areas come off easily with denatured alcohol. Any alcohol would probably work as well I suppose.

I use the Chrismar for traveling and that's waterproof anyway. The printed topo details I take are sprayed with crystal clear. That's not perfect either but they are just for marking up. That's mostly busy work. Even I can remember where things are 'till I get home and an done unpacking.