I recently removed an entertainment center that was built into an alcove that formerly served as shelving in my 1949 Bungalow. When the previous owner installed the built-in, they removed the plaster from one side to fit the width (including inside and outside corners) and two corners of the (triangular) top (also including both inside and outside corners) to fit the height. All of the framing remains. The plaster method appears to be 2-Coat as there is a wallboard base with about a half inch horse hair containing plaster over top. Metal lath appears to have been used to reinforce the corners.
The plaster surface finish is very smooth and I am not too concerned about trying to match the texture exactly (the planned repainting of the walls looks like it would add more texture than is currently present). As such I am not opposed to using drywall or the easiest most solid method.
> What is the best way to rebuild and repair these corners?
> How do I construct the proper backing/support for the repair and join it to the remaining "good" original material?
I would like to avoid ripping out and rebuilding the surfaces of the entire alcove.