Normally you can go to any outdoor store and they will have many dry bags to choose from. The one i use is for maps. It's slightly larger than my unit so i just fold the sides around back and tape them. It has a role top closure and never gets wet. Dampness isn't bad for them. It's the prolonged exposure to salt water that really hurts the rubber. Washing it in mild soap after sailing will do wonders for keeping the rubber soft. In a pinch i have double bagged it in ziplock bags. The regular kind not the gripper zipper type. Even though the gps isn't water proof, its water resistant. So you can use it in the rain or water ever. Just can't dunk it. Good luck!!

Reid Rechel