The Thecus is interesting, but the reviews don't give great confidence.
It's just a networked enclosure for two bays, and at that price, you could get a 4 or 5-drive Addonics MST or mini-storage-tower, with it's own PSU.
But then, you'd need to purchase the Addonics port-multiplier, exactly doubling the initial outlay.
Or, you could get a slightly-pricier DAS ready to go with the same features.
The OP is free to impose his requirements as he chooses, but he has a midtower "in-place" already. These DAS enclosures are pretty small -- just enough to hold the drives, PSU and port-multiplier. I'd say they're marginally smaller than a decent UPS box.
In another thread recently -- try "Power Supplies" -- we explored the possibilities with this, and the question is still open as far as I'm concerned as to whether you could simply buy a USB3 repeater cable to allow placing the DAS at some distance from the midtower -- with the same reliability.
But there should be plenty of NAS solutions. Since I have a server system with 4x 2TB HDDs, though, the DAS solution wouldn't require me to keep it running 24/7. Or if I did, it would still be networked storage at the server.