Hi - Your local glass works/glaziers will be able to cut a piece to your requirements. If you intend to fit this yourself it would be worth getting them to deliver it. Personally, I'd get them to fit a piece of glass this size as well. The responsibility is theirs if there is a problem.
If it's to be mirrored then get it 'safety backed', it won't cost much extra and it can be stuck back with the correct adhesive and avoid the need for fixing screws/clips (always a possible point of weakness)
As above, make a template for the holes, measure the outside dimensions carefully and go along to your local glazier. They will make it up and will recommend that it is toughened. We did one, which we wanted toughened and acid-etched (or whatever they call it). The glazier recommended that we fixed it to the wall with clear silicon but I reckoned that would look terrible as the silicon in contact with the opaque back would "wet" the glass and make it clear again, if you see what I mean. So we painted the opaque side with the same paint we used on the walls, three coats, then glued it on with clear silicon. I'm pleased to say that my hunch paid off and it looks superb, it has a nice greeny tinge to it, which matches the opaque glass door I got made for the bathroom cabinet I constructed.
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