I don't know what they're called, but they used to make simple rails for old work wallcases for mounting on studs in plastered homes which was just a hunk of sheet metal bent over on itself the depth of the ears on an old work box. You see them in this area in homes built in the 20s and 30s. Set the ears back, nail one rail up, prop the box in, nail the other rail on and you're set. Very secure. Maybe they still make something like that?
Here in SE PA I can't even think of one supply house where I've seen fiberglass boxes in stock. Most of the ones I've seen were in houses built back in the 70s and 80s. I think guys saw the issues with the screw holes stripping out and shied away after that.