" The cover plate in envision will have 4 screw holes: 2 for mounting the
plate
to the box, and 2 for mounting the outlet to the plate.

I presume because of mounting and strength issues, that this must be a
metal
plate. Which precludes it being a "trim" (ie, plastic) plate.

Any other suggestions how to accomplish this?

The cover plates that take a duplex outlet attach the outlet by a single
6-32 flathead screw into the center of the duplex outlet - the same point
that you usually attach a trim plate. I don't know about a round cover
plate for a single outlet (I've never seen one, but they probably exist)

You don't have to leave the metal plate bare and ugly. Degrease it with
lacquer thinner, spray on a good undercoat ( I like krylon) and then paint
it your color of choice. If the edges are rough, you can smooth them with a
file before you undercoat.

--Chuck