It seems a temporary, anonymous, private, receive-only dropbox(not the USB drive replacement kind) on the Internet is an unsolved problem. It doesn't have to be completely out-of-band like email, could be just an encrypted public reply by `cat | base64 -d | openssl rsautl -decrypt -inkey temp.key`, so long up to few bits(70 in this instance) of encrypted content would be allowed on a platform.