Given the conditions and size restrictions, perhaps you should be shopping for a piece of sculpture instead of a living plant? The small container size in hot sun is going to stress most plants. You could go for an interesting mix of succulents, but eventually they may become wider than you desire.

Kim is definitely correct that the size of your intended container doesn't lend itself to anything other than cactus or succulents. I would suggest you look for a glazed ceramic container more along the lines of a tall cylinder, perhaps 15 to 18 inches diameter by 2 to 3 foot tall for better success with a variety of plants. Euphorbia millii cultivars would work great, but these do have thorns. Euphorbia tirucallii 'Sticks on Fire' or Euphorbia cotiniifolia are other possibilities, or something like Sedum dendroideum v. praeltum, or Crassula argentea. You might also try Plumeria, if protected from winter rains.

Sansevieria (mother-in-law tongue) are pretty bullet proof. Mine thrive on neglect and can take full sun, but some varieties bleach out or can burn. I just shovel prune the sad stalks, as they are very prolific. I am not sure about frost, but a pot that size could be put in the garage, or in a sheltered place for a few months without harming this plant. I also think a 12" pot would be impossible to keep watered in summer.