I recently got a used QNAP TS-131P for cheap, that holds one 3.5" drive for offsite backup at a friend's house. It's compact and runs off a common 12V 3A power supply.

There is no third-party firmware available, but at least it runs Linux, so I wrote an autorun.sh script that kills 99% of the processes and phones home using ssh+rsync instead of depending on QNAP's cloud: https://github.com/pmarks-net/qnap-minlin