For most of these, the cutout sits just clear of the fillet (rounded portion) of the bulge on the back. That should (I think) leave about 3/4" or more of the rear of the faceplate sitting on the console face (or wherever you're mounting it.)
Take a piece of cardboard and an X-Acto knife, and start trimming away cardboard til it sits on the flat on the rear rim of the unit. That'll be your template.
The older units (like my LMS-332), as I recall, used a large square "U" of stainless bar with the legs braced against the back of the console, and a couple of screws into the unit through the "bottom" of the U. Cheap and easy way of mounting, I suppose; worked well, but didn't look all that sexy from inside the console.