Andy, did you check with Tractor Supply??? There are lots of 6 volt tractors still out there and maybe, just maybe they have what your looking for............if you get an extra 6 volt battery be sure to keep watch on it...or a battery tender on it.
I did and actually contacted another company “Wagan Tech“ to see if they might have something.
Yes- I would keep the second battery on a trickle charger.
