We had a custom mirror company custom fit and install a mirror in its place. This was very inexpensive, and even less expensive than a nice hanging mirror would have been ($80-100). We have this beautiful window seat across the hall from our mudroom and now its visually available from every angle and the mudroom appears much larger.
If you are doing this, make sure to trim the mirror first, then have them measure and prepare a quote. Make sure on the quote they include a polished edge on the mirror otherwise the edges will look unfinished.
This can also be done to replace hanging vanity mirrors in bathrooms to help increase the visual space.
We loved the result!!! All these little additions are helping our home look custom, cozy, cared about!