Chosen Solution
Can I fit the insides of a 2018 Mac mini into the Unibody of a 2012 Mac mini?
Bit of camouflage
How good are they going to be to know the difference? They are going to know it’s a computer because of the monitor and keyboard . Put it in an LC pizza box, no body wants one of those anymore.
How about finding a friend with a 3D printer and make a case as crazy as you can to hide it! A friend of mine 3D printed a copy of the Rodin sculpture ‘The Thinker’ a few years ago and altered the base to hide a Mac Mini on his desk as he hated seeing it!