Hopefully this works quoting the post of my battery switch install. Was working and wasn't able to find where in my mod thread it was when I posted earlier.
I only have one battery, so this may not work if you use two. All depends on how much room the battery tray has. Our 32 battery tray isn't wide enough for a 27 & 24 battery box.
SO I finally added a switch to the battery!!! Ordered a group 27 battery box, and a Blue Sea Mini Series single on/off switch. Just enough room to mount the switch in the box with a group 24 battery. I just cut a 2x8 to fit next to the battery to keep it from sliding.
I mounted the switch on the back side, facing the front of the trailer. Wanted to have it somewhat hidden so if someone decides to mess with the trailer, or worse, try to borrow it, they don't have any 12v power unless they find the switch! It is hidden pretty good unless you really look for it! Used a 2-1/2" hole saw, and filed the little bit needed to fit the switch (specs show 2-5/8" hole needed).
What you would see IF you could squeeze between the trailer and the battery box! Lol
From the side.
Hope this helps!