All you need to do is install an inline switch. Something which looks like this:
The one in this picture will work for 1000A intermittent, then 175A continuous. 1000A for starting is overkill. 175A will cover the rest of the time without issue. You can place this near the battery, somewhere inline on the ground would probably work the best. You'll want it out of the way so it isn't seen by anyone but you. You may have to figure out a way to mount it, but that isn't rocket science.
Something like this will only work for the 12v battery. You cannot expect it to work on the drive battery.
The issue you are still going to run into, though, is even with the battery disconnected, it will still drain down over time. Batteries lose about 2.5% of its charge every month. As the battery gets older, it discharge even faster. Without some kind of float charger (like a battery tender), the battery will most likely be flat if it sits for several months in a row.
Another thing to worry about is the fuel. It will go bad unless you put some kind of fuel stabilizer in it, which will keep it good for up to a couple of years if you use it correctly (and religiously).