Having experimented with the Raka both behind and in front of the rear crossbar, I decided to build a new wing which straddled the crossbar. I'm posting before I paint it, because it's easier to see what's going on if it's not black.
Features: It retains the ability to pedal if you want or need to. I can sit on it and pedal comfortably with the pedals set to 1.
It rests on the akas, so your body weight is supported when hiking out.
It's smaller and lighter than a Haka and works fine on its own, but it should complement Hakas very well. Combined with Hakas, it creates a continuous deck, making it easy to move from one side of the boat to the other.
It fits snugly and doesn't need to be strapped down.
Accessories: Here it is with hiking straps and an Ocean Kayak seat.
Normally the OK seat can just lay folded flat, but when pedalling, you need some support. For short burst of pedalling (such as when tacking), it's sufficient to just grab onto the aka braces. For longer periods, the OK seat is comfortable and provides support.
In principle, there's no need to dip your butt in the seat well at all. The one reason which I would have for using the Hobie seat is in turbulent water, where it can be comforting to be sitting down low where it's almost impossible to fall out of the boat. Fortunately the Hobie seat still fits, with the seat pegs in the front two holes, though I find I need the pedals set to 7
Technically, it's a quarterdeck, in that it's aft of the mast, forward of the poop deck (the rear cargo bay) and it's where the captain (me) controls the boat from.