RE: seamaster engine mount from plans?
I vote subfin. that's how they did in on the Lanier Mariner and the newer seamaster clone, the Neptune and Neptune II. I like it better with the sub fin, as my old seamaster's fold away one was always folding away. If I tightened it, it would stay for a couple of landings, then the balsa would give a little, and loosen up again. I supose you could glue a brass tube and rub plates to keep it tight, or add springs, but it's so much easier to just add the sub fin and extend the rudder down. The only reason you might not want it is for grass take offs. I sometimes drag the tail a little if I am doing STOL drills and on grass, it might hang up. In the water, it's kind of cool to come in on a low slow pass and drag the tail for 5-10 ft.
I have never hung mine on anything in the water. I expect it would do better than an overtightened brass rudder hitting something. At least there is a subfin to kick the tail up before something hits the rudder.
On the motor, that's almost how the electrics are mounted, but it seems like a lot of bother for glow given how many mounts are available. I would just buld light, and not sweat it too much.