Here is the battery tray. ITs $8 on Amazon and ships free. It seems raelly heavy duty.
Here is the battery selector. My advice would be to not run it in the "both" position, just switch between 1 and 2:
I got inline fuses, with quick disconnects at my local auto parts store, so i can quickly disconnect the sidecar battery cables. They were about $10 for 30 amp fuses in a 4 gauge wire size.
I got the battery at a local battery shop. $100 buys a decent deep cycle marine or RV battery. I ordered the stuff yesterday, and went with size 27 battery. The batteries are much cheaper than the large 31+ sizes. They will still run pretty much all of my camping stuff and my bike. I havent installed it yet, its a winter project.
The battery weighs 55 lbs, so its enough ballast. I can easily take it out when I dont want it, since the tray has a quick release strap, and just set the battery selectopr to use the original bike battery.