Craig, I’m not a professional plaster, only a die hard doit yerselfer with some electrical training. However, over the years I have had occasion to try different techniques of accomplishing the patching you are talking about. One good thing is if you don’t like the looks of the method you tried, you can always cut it out and try something else. And yes I have encountered the construction you are talking about in a house built in the early 1950’s. My presently preferred method is to cut the hole all the way out, back to the center of the closest ceiling joist. Carefully measure the thickness of the plaster in several locations ( it will vary ) and come up with a combination of wood shims and drywall to get back as close as possible to the thickness you need. Then use drywall tape, joint compound, etc.. to finish the joints. Good Luck. Take your time, and have fun with the challenge. BobB