Couple options. There's a mounting bracket let's call an adapter ring that you fasten to your two box's holes, then mount your fixture anywhere on the ring. Another type bracket incorporates two linear brackets, joined at a pivot-point in the center. Hard to describe in words, so I suggest a trip to HD/L; they'll know what you need.

Lonesparky,

Thanks for the reply......I'm off to HD for mounting brackets!! Thanks again.

Alida

What you need is called a cross bar. EVERY fixture comes with one to enable you to mount to almost ANY box.
The holes in the fixture have NO relevance to the box itself.

Petey Racer,

I agree, most fixtures do come with a cross bar. Unfortunately, these don't. The backplate to the fixture is a flat, circular shape, meant to sit tight against the drywall. There is a large hole in the center for the wires but the holes in the backplate for attaching it to the box are clearly positioned for a square box. No cross bar. And the instructions are pretty clear on proper installation. I wish manufactures would include installation requirements with product sales info so folks like me could be sure of a compatible installation before we get it home and out of the box..... but thanks for the reply. All help is most appreciated.

A brand name Angelo offered an extensive line of bars and box adapter plates for fixture mounting. I do not know if they still are in business by that name.

Does it matter if the box is plastic or metal for a fairly heavy sconce?

GRRR

the reference to poor instructions...

I installed an outdoor light fixture once that came with NO instructions whatsoever, and showed no clear way to install it. There was a non-hollow "arm" that held the thing up, and a hole in the bottom of the light itself whose size didn't match ANY standard wiring device I've ever seen.

Code expects me to install things as the manufacturer intends but I'll be darned if I could figure out what kind of drugs the manufacturer was smoking when they engineered that thing! I ended up modifying the thing severely. Redneckville (re-)engineering at its finest.

How can you replace a rectangle sconce with a round sconce when the base box in wall is rectangle? It leaves gaps around the base. All the new sconces come with round bases?…

How about drilling some holes in the sconce backplate to match those on the box?

Hopefully, the OP resolved this 13 years ago...