Do you have to use that particular wall enclosure thing? A 1U chassis at 1.7” of height fits 4 drives (and a 2U at ~3.45” fits 12), and something like a QNAP is low-enough power to not need to worry about cooling too much. If you’re willing to DIY it would not be hard at all to rig up a mounting mechanism to a stud, and then it’s just a matter of designing some kind of nice-looking cover panel (wood? glass in a laser-cut metal door? lots of possibilities).
I guess my main question is, what/who is this for? I can’t picture any environment that you have literally 0 available space to put a NAS other than inside a wall. A 2-bay synology/qnap/etc is small enough to sit underneath a router/AP combo for instance.