On the standard 403, I accomplished this by mounting it just aft of the rudder and offset to one side. This way, the rudder is free to move in all directions including up and down.
Since the 403AC does not include a rod to extend it outward as does the 403 and 403C, you'll have to devise your own. Perhaps a flat metal plate to get it to extend beyond the rudder. It should be doable with a little fabrication.
BTW, if you're considering the 403AC, you should also look at the new
1103AC which has 3HP rather than 1HP and doesn't cost that much more.