Hard to tell exactly how big a hole you are dealing with. Maybe a little wood putty?

Joe

It's less than an inch around, it's a terrible picture! It turned on it's side when I uploaded it. the hole goes into the outlet box.

Keep in mind, bugs are quite resourceful and will find a way to get into just about anything they think worthwhile. Ants are particularly drawn to electricity for some odd reason.

One inch is probably a bit big to fill in with wood putty. So then I would suggest finding some thin, stiff material (some thin hobby board, metal plate of some kind, plastic, whatever) that you find aesthetically pleasing or can paint to be so, and goes with everything the way you like and create a backplate. If you want to go that next step you can seal underneath/between the backplate and the steps with putty or silicone.

Depending on how regular the hole placement is (like if it is always on top or on the side) it can be just big enough to cover that one side. Or it can be big enough to look like a mounting plate or edging for the whole fixture. Use your imagination and aesthetic taste to suit your need.

I don't see coming up with something that will disappear, so best to make it look deliberate and belongs, even if minimal.

That's my suggestion.

Joe