After removing an old bath vanity light above the medicine cabinet, I was expecting the electrical box behind it is not a typical octagon box or a square junction box with a round mud ring. Instead it is a metal rectangular switch box.

The new light I have, which is a 4-lite vanity light comes with a round mounting plate which normally would account for variations in distances between the two mounting holes on the box itself. However I am having trouble getting it to work.

The instructions calls for the round metal adapter to be secured to the electrical box with two screws. Then there are two screws that are to be mounted so they point outwards to go through the face plate of the light fixture. For this reason, these four screws (two in, two out) must be aligned in a straight line.

<img src="https://i.postimg.cc/x1fC7cmZ/IMAG1327.jpg" width="576" height="324"/>

<img src="https://i.postimg.cc/50CNvvcX/IMAG1328.jpg" width="576" height="324"/>

This is the round adapter plate that comes with the light fixture.

<img src="https://i.postimg.cc/rpbwKjH6/IMAG1329.jpg" width="324" height="576"/>

In the picture below, I threaded the two outward pointing screws that the light fixture is to be mounted on. However, the spacing on the switch box screws are such that they will not fit on the slots just outside of those two screws.

<img src="https://i.postimg.cc/qv1Rd4Xv/IMAG1326.jpg" width="324" height="576"/>

OK, so I rotated them around a bit to find where they may work, but the problem is, these are bigger longer slots, and when I mounted the fixture on it, the fixture having four lights, it's heavy and therefore it shifted in the slot and the light shifted out of horizontal.

<img src="https://i.postimg.cc/rpzpV3QC/IMAG1330.jpg" width="324" height="576"/>

Any idea if there may be adapter plates I can get to make this work? Or is it better to just get a metal strip and drill and tap the four holes precisely where I need them at?

 

I've run into that problem many times in the past. I re-drill the round plate where needed.
You could customize your own plate.
You could also use nuts and star washers on the fixture bolts and they will stay where put.

 

The mounting holes do not need to line up with the studs.