How much do you want to spend?

If you go for a Raspberry Pi have a look at Terrapi cases as well the obvious Argon ones.

Another option would be a Zimbaboard. It is more expensive but it has dual SATA connector (you need to buy a Y cable with the Zimbaboard) and there are 3D print designs to create a single unit, e.g. printables.com/…/224057-zimaboard-dual-hdd-stand.

I’m not sure about PoE and a NAS. Will a PoE HAT or similar provide enough power for the board and the drives?