Make a polished stainless steel escutcheon plate or a solid surface mounting block are about your only options I can think of.

There are many commercially available items for you issue. I used to call them shower trim plates, used to cover gaps such as yours. But, I believe the correct name is as above; escutcheon plate. They are also called; shower cover plates.

I did look at a few. The problem is finding a matching finish, which in my case would be polished nickel, which varies from manufacturer to manufacturer. What are your thoughts on having a glass/mirror fabricator make a rectangular mirror as a back plate? Is that feasible. I have seen sconces mounted on mirrors.

If you go with a mirror, then it has to have mounting holes drilled through it and a large central hole so the wires can be connected. Doable, but it would be a fragile piece that could crack when the sconce is screwed down on top of it. I know you are concerned that someone else's polished nickel won't be identical to your polished nickel, but the reality is that most polished nickel finishes are close enough that other people would never notice, and you'll quit noticing after the first week. I'd go with the simple (and cheaper) solution of an escutcheon plate.

It's not just the finish that has to match (I think they are all close enough though). You want the edge profile to match the sconce's if it's metal. It's a simple profile, but it is distinctive.

I don't suppose you have any tile left over...?

There isn’t enough tile left over, and there are 5 sconces in the room, so it would involve quite a bit to tile work. My project manager, who is a carpenter, suggest a wood piece to match the window frames & medicine cabinet but I think it would look too clunky.

I spoke to a glass and mirror shop and they suggested custom acrylic mirror instead of a glass mirror (less fragile). I’m leaning towards this option. How large should the mirror plate surrounding the sconce’s backplate be. Will one inch around the perimeter work or should I go biggger?? Pencil polish edge or beveled?

Acrylic MIrror

I just love how a fixture that came from a kitchen and bath remodel store have 3" plates and no one told you that upon purchase.........

@RL Relocation LLC "I just love how a fixture that came from a kitchen and bath remodel store have 3" plates and no one told you that upon purchase........."

The first item of the Installation section of the Installation Guidelines states that a 3" x 2" switch box is REQUIRED (caps theirs).

Yes, I goofed. I supplied most of my light fixtures and didn’t run these by the project manager or the electrician.

wdccruise thanks now i know.

I would contact the lighting manufacturer and ask if they have anything that would work. If not I would get more tile and make the holes smaller. It would bug me to always look at my lights and be reminded that I had to alter them to make due. I do really like your selections.

Can you get trim pieces that go with your particular tile? If so, you could frame your sconces with tile, kind of like this (disregard the circle tiles):

@skfso: I contacted Waterworks - they had nothing for me. The existing tile is 20 year old, handmade subway by Meredith, no longer in business. I tried to match it when we redid the shower and we were unsuccessful. Besides, chiseling tile off the wall for repairs is risky. The entire room (floor to ceiling is tiled). Therefore, tile work is out of the question, which is why I need a hack.

If you go with the acrylic mirrors, I would make the "margin" outside the baseplate the same width as the vertical part that holds the bulb.

Yes @Hot Rod! This part?

A backplate in the white glass to match the shade would be ideal.