I know I'm bringing up a really old topic, but I wanted to simply correct something I saw.
You might want to allow some air flow under the door as you'll be forcing air in from the AC units, so the pressure has to go somewhere.
Window A/C units don't force any air into a room. They only strip the
heat from the air already in the room and dump it out the back. I can't see any pressure increase from that, though I might be missing something.
Seal 'er up tight.
The rest of your post and the others here have been really informative, though.
If I had the space and a little dough I'd do it. Stainless steel panels on the inside walls, which would be constructed of SIPs would do the trick. Elevate the "room" and insulate the floor to the same degree as your walls and ceiling, and run a drain out through this 12" joist space, too, for the mess-factor. The floor could just be epoxy coated tongue and groove flooring boards, sloping in (or not, just squeegee) to the center drain. A nice door could be made from another sip cut to fit and hinged, weather-stripped, and latched. You'd need to seal the edges of the SIP somehow, too, probably just adding to the stainless panel that makes the interior side of the door. Hell, while you're ballin' out, why not fabricate some kid of duct manifold coming out of the A/C unit supply openings that more evenly distributes the air throughout the cooler?
I wonder if building one like this would be cheaper than just buying a prefab walk-in for about $6500 but I imagine it would be. And based on a google search worth of info gathering, I'm seeing R30 as being about average (could be wrong there), which could easily be beaten by a custom build. (Might not use SIPs though because they are something like R14) Although the marginal benefit of adding more insulation is probably beginning to go down at that point, your bills would still show it.
I wonder what ever came of this plan...
Take care of yourselves