Lots of opinions on the exact placement of the cavitation plate relative to the hull... some say slightly above, some say slightly below. I found that going perfectly level yielded pretty darn good performance (much better than several inches below, as youre describing). I found one of these T.H. Mini Jackers on ebay and its a nice piece. I use it for the 15hp Johnson on my 12' aluminum boat.
http://www.boatersplus.com/t-h-marin...FQqe4AodXQ0AgA