Umm, I think someone has been confined at home too long.
What you are describing is not “(small)” for personal use.
How about a freestanding humidor hutch placed in the opening? In your taste of finish, of course.
Save a lot of headaches and you get the instant gratification and use. If you ever move, you can take it with you.
With that being said, your existing painted drywall will hold moisture fine. No vapor barrier needed for that. The question is if it will resist mold at 70ish % RH, and I think at that level it probably will be OK. Cigars, a natural fermentable product doesn’t support mold until above that.
Not necessary to line with SC. ..$$$ Unless for aesthetic or tradition reasons or for that SC aroma. If desired, you could line it with just about any wood; oak, maple, birch, poplar, pine, etc. Just not aromatic cedar. A thin veneered product would be the most economical and easiest to install. Could also go with ship lapped pine, if that is your style.
Your biggest challenge will be sealing the opening and providing humidification. That size, you may need something along the order of an electronic humidifier with humidity feedback. You already have the power supply in there.
I do like the sliding glass door or French door option, but a little pricey. You could install yourself, but your opening will likely require something custom as it is a bit narrower than the standard. If you go with a slider, it needs to open automatically with a wave of your hand and go “whooosh” like on the Enterprise in Star Trek.
Oh yeah, almost forgot. Lighting, you will need lighting, even if it is plug in led strips or rope lighting. A GFCI is overkill as you won’t be expecting open water.
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