

You just stick the magnetic backing to the existing glass plate for flatness and rigidity. You might have to bump the heated temp 1 or 2 degrees to compensate for the additional thickness, and you may lose a couple of mm/s of print speed at the high end due to the additional weight. That said, it is a very cheap and easy to execute option.
To answer your question, yes that PSU looks like it should work. I’m not sure I would use this PSU for my NAS due to the quality of the manufacturer being unknown, but generally it should be fine so long as it is not mission critical.
Also if you have access to a 3d printer, there are some great designs for external drive cages to keep them safe and cool.