The absolute cheapest way to get storage. For the barest bones, ugliest setup, I've seen people have two PC power supplies on their desk, a motherboard, a few SATA cards, and a number of drives plugged into the SATA cards, and using Linux + ZFS + Samba for the heavy lifting. Alternatively, a "NAS PC case" with a decent motherboard and such should work.
If I were building the cheapest way to have a lot of storage, but have a warranty, I'd go for a higher end QNAP NAS that supports QuTS Hero, even QES. I would then load TrueNAS SCALE on the QNAP hardware, use ZFS from there on out. This ensures a lower attack surface, and ZFS without any added stuff. The QNAP hardware isn't cheap, but it is fairly reliable.
Terabox only allows 20 files to be saved on the free plan. Each file can be a max of 4 GB, which means that you can only store at most ~80 gigs. That is the main gotcha.