Depends on your budget and goals. Lowest barrier to entry; use old college textbooks books in a pile or an old Amazon box to sit the mirror on. For a bit more, you can buy a fancier box to sit the mirror on. For the most complicated solutions, you could use a picture rail like this: https://images.app.goo.gl/dYsZVtMVny1k7RTx9 or suspend the mirror from the ceiling. Like these bathroom mirrors: https://images.app.goo.gl/p4cAMC6yjCZyypoB8 or like this: Della Round Metal Wall Mirror https://www.wayfair.com/decor-pillows/pdp/gracie-oaks-della-suspended-modern-contemporary-distressed-accent-mirror-w005060148.html I would definitely think twice about blocking an HVAC cover as your system may not run efficiently and you’ll end up paying $$$ in bills or apartment suing you for ruining their system. More on reddit.com

My cat would run face first in to that and knock it over but it looks nice. I’d get it for $150 More on reddit.com

looks so much like an arched doorway. love it. also, someone is going to walk right into it :D More on reddit.com

I used to order these for my work in physical therapy gyms, never found one cheaper. https://themirrorcompany.com/ It's a 6x8, $299. They'll cut it to your specs, and mount it wherever you want for another $100. Anything over 3 feet is generally in Gym Mirror territory, and that's what you'll likely search for to find one that big. More on reddit.com