2 pc of plywood, to make a form for each side,
wire, as in steel bailing wire,
a pound of 16d framing nails,
a section of 1/2"cpvc pipe.
Cut the pipe in lengths to match the thickness of the wall.
Hold 1 piece of plywood up over the hole and trace an outline of the hole on it. Top edge flush with the top of the wall!
Stack the two pieces together with the top edges flush.
Drill 1/4 to 1/2" holes in the area of the hole you traced about 8" to a foot apart. Make sure a couple of the holes are close to the traced line and near the ends.
The fun part:
Add up the distance between the face of the plywood, the wall thickness and the other piece of plywood.
Cut pieces of wire 3 times that long, fold it in equal halves. You will need to make one of these for every hole you drilled.
Hold the plywood up on it's edges and push the loop end of a piece of wire through a hole in one piece of plywood, through a piece of pipe and out through the matching hole in the other piece of plywood slip a nail through the loop and with another nail, twist the wire together on the other side and push a nail through the wire and use it to tighten the sides together. Just do a few, a couple at the bottom and one at the top.
Slip the whole thing over the wall and install the rest of the wire and pipes with nails to hold it together. There's your form, fill it with bag mix and cut the wires the next day, pry the plywood off and fill the pipes with caulking or mortar..
You may have to improvise or get a helper if there is a roof over the top of the wall
Good luck with it!