After marking the location for the junction box with an outline, use an oscillating tool to cut the plaster (not the lath).

FOR some time past I have been observing the streaks which occur upon lath-and-plaster walls. I have made a survey of the literature and find no adequate treatment of the phenomenon. For that ...

It’s more difficult to cut into plaster walls when you need ... Toggle bolts are an option in plaster walls with metal lath when no stud is available. Drill holes for these carefully to avoid ...