In an enclosure that short, I'd put in at lease two heads, one at each end (or one in the middle and one on an end). You can adjust the mist, but 12 inches is not a lot of space for it to spread before it hits the ground. You can install one, then watch how fat it goes then make a decision from that. Newspaper in the bottom should give you a good indication of where the water is hitting from the wet spots.