Pursuant to the 'boat tail trailer':
I did this for another discussion. If you separate the load into two lumps you could have a seat in the middle of the boat. I don't recall if the discussion touched on whether you'd want the boat backwards. Like this:
For the battery packs what you want is 'hot swappable', a controller that will tolerate intermittent sources, maybe with a configuration switch or [soft] buttons. Does Litthium-ion have better standby life?