It seems your major problem will be maintaining the moisture without spending a lot of time watering manually. Every time I have put myself in a position of daily maintenance by hand I have regretted it. Is there no possibility of installing an automatic watering system? Al

I just rent and my landlord will do nothing with the building.The best I can do is a rainbarrel and my two assistants. Its frustrating at best.

Can you possibly unobtrusively bury gallon jugs with small holes poked in them in spots in your containers? I am trying that with my raised bed, very hot sun, extreme S. TX garden this year. And some of my containers, with smaller bottles. As for plant ideas, try maybe High Country Gardens? I know they have Xeric stuff and a very nice website.

No problem burying gallon jugs, most of the perennials are mature enough that the foliage will hide them pretty quick.
Placing them and filling them with gritty mix seems like it will be a real trick. I am assuming that putting "filler" in the bottom third is not a good idea? I have been reading previous posts, but there are ALOT of previous posts.

First, put in insulation such as styrofoam pads in the metal box. This is because you need to control not just moisture but also temperature swings (in winter and in summer).

Second, put in PVC pipes (perforated near the bottom) to let the air into the box and for when you water the plants.

Third, is there drainage holes at the bottom of the box? If not, put holes in preferably at the bottom side of the box.