This may seem an obvious /silly question, but bear with me because I want you to consider it from a pragmatic view point.
I have a fledgling project that will be mains powered and fit in a space roughly 200mm in any dimension. Initially you'd think that a plastic box would be suitable. But consider: There will be penetrations in the form of a switch and some sort of optical indicator, both perhaps with metal bezels /lever. And there will be a mains transformer, with a bolted attachment through the case.
What initially seemed a simple example of a Double Insulated box, now has metallic (conductive) penetrations though it, compromising the insulation integrity. Commercial Double Insulated kit is quite complicated, with blind self tapping screws, multiple insulation layers, e.t.c that all seem a little out of reach for the typical electronics enthusiast.
It seems to me that it's safer to treat the whole thing like a washing machine, and put it in a metal box with a good earth ...
P.S. Should I look for Aluminum or plastic enclosure for UPS touches upon these issues, but does not seem to address common enclosure penetrations.