New answers tagged stucco
The simplest way to stucco the opening is to use cement board only because you'd be saving several steps by not having to apply each successive layer. Looking at your photo it appears that you have at least 3/4 inch of depth from the stud edge to the existing stucco surface. If you install 1/2 inch cement board over 30 # felt you just need to float a 1/4 ...
The important part is breaking out the old stucco to create a rough edge, and expose the wire to tie into. Sheath what you have with wood. Slip new paper (two layers of grade D in the USA), under the old, and wire. Now an alternative is to cut cement board in the shape of the old window and caulk the edges. But here you'd want to recess the cement board ...
You can use any of those materials, or none of those. Waterproof paper (properly installed such that water is prevented from penetrating the repair seams) and wire mesh alone, with no substrate, may be just fine. An important part of the job that you do not mention (and is not apparent in your picture) is the waterproof paper/membrane.
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