Here's the 3D, and the profile in Profile Editor that I used to create the niche.
Nothing is parametric here - so a niche-per-wall thickness would be required with this method.
I made a profile that was the thickness of the wall or deeper - to cut a full hole, rather than an actual niche. Then, I drew a beam on the floor (bottom of beam at height zero), set the ID for the beam to be "wallniche" (no quotes or spaces). Finally, select the beam and view only it in 3D, File > Library save 3D as object, select Window and make sure the Options are set to editable GDL. Give it a useful name.
With the Window tool, place the niche object you just created in the wall. Now place the desired window in the wall and drag it over the niche to get the desired final assembly.
Cheers,
Karl
PS If you need the header curved, too, you'll have to model that mass before saving it as a window (or door) object with ID wallniche...
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