I have cut out two holes for 4", metal electrical boxes to be set in to the exterior walls of an old block and stucco garage. I need to secure the boxes by setting them in either concrete or mortar, but am unsure what type of product to use. Would a plain old Quikrete - their basic concrete mix - work for this? Or should I use some kind of mortar mix? I was about to use the common Quikrete simply because I already have some, but then noticed on the package that it said it should be applied a minimum of 2" thick. I don't have that kind of space around the boxes, so I thought maybe that was the wrong product for the project. I also happen to have some old thinset for tiling. May be past its prime, but might work for this? Of course, I'm not opposed to buying whatever the right thing is if neither of these are it.
Any help would be appreciated.