My “NAS” is a “slim” Intel NUC 8 with a 3D-printed “riser” that allows it to fit a 4TB (15mm) 2.5” HDD. No redundancy of course, but otherwise it’s superior to a legit NAS or Pi NAS in every way.
https://www.thingiverse.com/thing:5441566
There’s also this one that I did not try, which might fit 15mm drives a little better: