i made a transom for my canoe by drilling one hole in each side just below the top in the metal strip and used a muffler C clamp on each side, drilled 2 holes in the 2x4 on each side. looks like you could easily bolt through your deck and mount the 2x4 flat. if you can i'd bolt a section of 2x4 to it standing on it's side for the actual transom. nothing to it.
Classic Freestanding Rack
Touring Kayak Paddles