I had new receptacles installed in my existing office. Therefore, an old work box can be used (one of those electrical boxes with wings that flip out when you tighten the mounting screws... it then holds onto the drywall). In my case, the hole was cut too big. I found the problem as I unplugged a computer from the wall and the whole box + receptacle came with it.

This thing is ugly, but it works. Youtube videos suggest cutting an entirely new hole, patching the old, sanding, painting, and trying again... but I am too lazy to do that. It uses 1/2" spacer surrounded on both sides by larger 1/8" plates. The 1/2" corresponds to the thickness of drywall in my office.

To install, I pulled the box out of the wall swung this device into place, and shoehorned the box + receptacle back into the hole. Everything tightened up nicely as I tightened the wings in place.

To make it prettier, print it in a color that matches your paint, or try to modify the edges such that the visible "lip" that overhangs is smaller and have a bevel that matches the inside of the wall plate to allow it to sit flush. In my case, this one is hidden under a desk and no one will see it.

Good luck!