My wife purchased some new sconces for a room in our house. We're remodeling the room and I put round boxes in the wall in anticipation of the sconces. When the sconces arrived, they have a 4" X 1.75" box in them, with a mounting bracket that is 1.75" wide, horizontal to the light fixture.
What type of box do I need in the wall to accommodate these fixtures? I can't seem to find anything obvious. Alternatively, is there an adapter for a standard box (rectangle or circular) that would let me mount this?
Your fixture appears to be designed to go over a standard single gang switch box, but the mounting crossbar is oriented the wrong way for doing that. As a result of that, and the fact that flat mud rings for 4" round to single gang aren't a thing, we'll need to get creative. The first part you'll need is a Satco 90-110 or equivalent 4" nominal crossbar, screwed to the existing crossbar on the back of the fixture to serve as an extension. From there, you can then attach the crossbar to the holes on a Garvin 54C41 or other similar 4" octagon/round cover for a single receptacle, mounted so that the holes are lined up horizontally. Note that you may need to mount the cover with the aid of a universal crossbar to get this to work.