Fill large gap in plaster with no lath
Roll up some fiberglas insulation like a jelly roll, so you can stuff it into the opening. Keep it recessed about one-half inch, so it acts like a foam backer except non-combustible. Make it a tight fit. Remove the front cover if you prefer, and tape a piece of heavy plastic over the electricals so you can just access the gap.
Fill the new gap with plaster or drywall compound, flush with the existing wallboard. A masking tape barrier will keep the mud from touching the panel box. Re-install the front cover, then sand and paint to match.