My neighbor buried his two barrels halfway and his water stayed clear all summer, so shade plus buried is the move.

Kinda have to disagree with blaming the tender here. A 68 Camaro has zero computers and basically no parasitic draw, so the battery will sit for months and hold a charge fine on its own, you only actually need a tender if there is something draining it or you want it topped off once a month or so. Leaving a cheap charger plugged in for four straight months, unattended, in a metal shed with no airflow, that is not a battery tender problem, that is a guy who never once walked out there to check on it. Even a good smart unit with temp cutoff will fail if it is a junk brand, so the real fix is a decent maintainer plus actually looking at it every few weeks. And honestly the fire alarm point is the thing everyone skips, a 20 dollar smoke detector would have saved him the whole mess.