I wanted these to exist so badly, it was a fun and quirky concept, but people kept bashing it online as the stupidest idea ever, and the few dead drops that existed in my city routinely got destroyed.
I also built my own PirateBox too, but the only thing that was ever uploaded to it was a creepshot of me and my PirateBox. Turns out when Public Wi-Fi exists, they're actively ignored.
The dead drop website had a map but I was never anywhere near any. That sucks that people destroyed them. It's a dangerous idea, but a cheap used laptop specifically for it could be fun, but yeah, not enough people would ever do that to make it worthwhile.
USBkill, etc.
Wifi captive portals with simple file sharing are probably the safest way to go. You could also locate it in a hard-to-reach place, or completely out of sight, to make it hard to destroy. My favorite idea is to fit a tiny ESP board inside an outlet box.