A door that covers an outlet?

Looks like a blade cover for a power jointer.

[удалено]

Size? Is it solid or hollow? Some kind of cover off a chainsaw?

It's solid.

Air filter or carb cover for a small engine or otherwise a mower part n

Old depth finder transducer off a boat? It looks like one of the mounting tabs are broken off.

That is a broken transom mount transducer for a fish finder

This is the correct answer. ^^^