Ideally it would be in the back of the wagon, but they stink, drip salt water, make a mess and generally drive you nuts.
A twin rear wheel carrier with one side converted to an outboard carrier would do, but you'll need a real good engine cover, and the better they get the more the motor sweats inside which is obviously unpreferred. 4wd systems do these rear bars with outboard mounts.
A tinny and 15 h.p. motor on the roofrack will kill your gutters and overload the roof.