What do flanged power inlets attach to?
Flanged devices like that would usually be mounted to a piece of equipment, a hole would be cut in the panel of the equipment and then the flange would be screwed/bolted to the panel .

Physically you could almost certainly cut a hole in a blank box cover and then bolt the inlet to it. Whether that would comply with regulations in your country or whether you need to look for an inlet that is specifically designed for installation rather than equipment use I could not say.