There is a really neat design for a folding dink of about 11 ft in
Wooden Boat #123 of April 1995. Best for rowing but will take a small
outboard. Plywood design, but maybe could be translated into plastic.
Folds to 4" or so, looking like a surfboard. Transom and seat
assembly (essential for spreading sides and bottom) may fit inside,
but I couldn't discern that from the article. Very detailed "here's
how I did it" instructions and good pics and illustrations both.
Quite an achievement, IMHO.