Do I need to use a metal box for an outlet? Answered

here's another idea. I'm not saying it's a good idea, necessarily. Cast a waterproof extension cord into the piece. Again, I ask: how much power are you drawing

Metal is best for short protection.

There are receptacle boxes made for imbedding in concrete both plastic and steel.

Ether way use a GFCI receptacle.

how much power are you pulling? The main caution may be accumulated heat.

there are also plastic outlet boxes. If you must modify it, go with plastic. Metal can short out your outlet.

what is the advantage of omitting the box?

The metal boxes allow you conveniently remove the socket plate from the wall.

This makes it easier to connect the wires, and also makes it possible to renew wires without having to dig holes in the wall. For the same reason, you should have your wires in conduits (pipes, tubes) or other open spaces inside the wall, so that they can be renewed without having to destroy the wall.