Our vanity light consists of a long row of 6 bare bulbs with a metallic coverplate stretching a total of about 4 feet. We're replacing it with a newer, more modern light kit.

I removed the old light and found there was no electrical box in the wall. The old light kit was instead secured by two wall anchors a couple of feet apart and the wire was left dangling in the stud bay.

The hole in the wall is irregular (I assume thy weren't too careful because the cover plate was so large) so there's no place near the center of the vanity where the tabs of an old work box will be able to attach themselves. There's also a vent stack in that stud bay so I can't use on of the electrical boxes which come with extendable arms.

The only thing I can think to do is attach an electrical box to a small board, put some construction adhesive on the board, and slip it through the hole so it's glued to the inside of the sheetrock.

As an aside, the lighting fixture isn't that heavy - maybe ten pounds.

Would need to know exactly what the new fixture looks like, esp the portion that will sit on the wall surface.