An unattended ballot drop box should be no different than an unattended USPS drop box, either were acceptable for submitting my mail in ballot. My drop box was at town hall, right next to the fire department. I hope nobody would be dumb enough to start a fire there.
There have been some reports of someone tossing a flaming object into the boxes. At least here in California, best practices (maybe even a requirement) are for a rigid, narrow slot. I suppose it makes it harder to toss in a larger object that's on fire or maybe a Molotov cocktail. So no repurposing surplus USPS mailboxes.
Some are at indoor locations with limited hours. Not sure if they would have a fire extinguisher nearby. I've heard that some of the 24/7 drop boxes have automated fire suppression systems. It might create a mess, but they would likely stop them before the ballot are damaged beyond recognition.
In Ada County, ballot drop boxes have a fire-suppression system designed to prevent exactly that kind of problem. They’re also monitored by cameras 24 hours a day, Chelsea Carattini, spokesperson for the Ada County elections office, told the Statesman.
“We have installed two fire suppression devices in each drop box,” she said. “The device triggers a fuse that suppresses fire when it comes into contact with a flame.”
That article references this one:
Yet even some counties that employ different types of ballot boxes have borrowed other security ideas from Pierce County. One of those was using chemical fire suppressants inside the drop boxes to ensure ballots don’t go up in flames.
Anderson said firefighters in Pierce County helped put the fire suppression system to the test, tossing lit cigarettes inside, as well as pouring in gasoline and lighting a match. While a couple of the test ballots got singed in the process, they were still legible.
Mason County Auditor Paddy McGuire said he initially tried to crawl inside the county’s ballot boxes to install the fire-suppression system, but had to find another, smaller volunteer to do the job. (“Drop boxes are not designed for full-sized human beings to climb inside,” he recently said of that effort.)