Years ago I worked as an residential electrician's helper in high school and college. About 10 years in total.

If the light fixture is heavy you may want to place some thin metal/plastic stripping between the ears of the new Old Work Box and the sheetrock. Heavy fixtures tend to do exactly what this one did over time. Placing something rigid between the sheetrock and the ears prevents this by increasing surface area and keeping the edge of the ears from cutting through the sheetrock.

Most hardware stores sell some sort of flat strip with holes in it. Cut the strip to length, and use a finishing nail (very thin, easily pushed through/removed from sheetrock) to hold the strip in place while you place the box.

NOTE: Make sure the nail will not mar a wire.

TIP: Try to Find an area that will be covered by the fixture and place the nail there.

Once the box is installed and the ears are holding the strip in place, remove the nail.

There are other methods to keep the strip from falling into the void, but this one is easy if the holes can be hidden.