You can mount the battery cutoff switch just about anywhere. Some people mount them on the vertical sheet metal under the seat so it can be reached from the driver's position.
Here is a link to a marine grade double pole battery switch. This switch will disconnect both sides of the battery, both ground and hot sides.
Nkaminar- Thank you for the link. Using the brand and model number it’s listed for a variety of prices from different sources, and shipping costs differ too. Looks like another spreadsheet is in my future.