Context: I got it in my head that I really REALLY wanted a doorbell. I have light switches by my frontdoor (shown in image), and a porch light directly opposite them. Because of the placement of the porch light relative to the light switch, and some tapping on the wall, I could tell that the space above the switch was hollow, and I assumed that my electric lines passed through that way. I have a doorbell transformer that I need to install - I figured I would open up a space to the left of the switches for a box where I would install the transformer. Well, I started cutting out the drywall when I discovered that nope - that’s where my electric lines are (I damaged the jacketing on the cables, but I should be able to fix that with some shrinkwrap heatshrink jacketing, I think). So I broke open the hollow space, found that once upon a time, there were electric lines there, but they’ve long since been cut. So, at the least I now know where I will install the transformer, and should otherwise be able to get this doorbell installed.

My question: After I’ve finished the install, I don’t know how to close up these holes I’ve made - or more specifically, I’m not sure what product is right for the job? I imagine some sort of plaster? Or mortar?