Here's a link to the one I installed:
Amazon.com: Marinco 701 Battery Disconnect Switch: Automotive
The switch has three removable sides. I used clear silicon to seal the sides and mounted the switch on the battery box with the open side down.
Because of the variety of ways in which this switch can be mounted, it does NOT come with mounting hardware. I bought stainless bolts just long enough to fit into the battery box cavity, and secured them with stainless washers and lock nuts.
You will also need to buy a short cable to connect the switch to the battery. The cable that is currently connected to the battery will connect directly to the switch. As best I recall, I connected the switch to the battery's positive terminal.
This is a great mod! It's relatively cheap and easy, but with big benefits. When storing the camper, I used to remove my cover, disconnect the positive cable, and replace the cover. To prep it for camping I had to reverse the process. Now I just flip a switch!