Check this out. This might help you decide what size AC unit to get. It has specifications for how big of an air conditioner you need to cool a certain area. There also a video on how to install the AC unit. It looks like they have the AC unit up high.
https://www.storeitcold.com/build-it/ac-selection/They also sell prefab boxes, but I'd build it yourself. They're only a cool $4200.00.
https://www.storeitcold.com/coolbot-walk-in-cooler/
