here's what I know about sof:

http://home.pacifier.com/~qayaq/index.htm

http://www.arctickayaks.com/plans.htm

http://mind.dreamhost.com/kayak/

http://mind.dreamhost.com/kayak/build.html

http://rtpnet.org/~robroy/baidarka/slideshow/node-o08.html

http://elwood.gpi.uni-kiel.de/Kayak/

http://www.pouchboats.com

http://www.pouchboats.com/building.html

www.imaginelan.com/aleut/./polyester_skinning.html

you probably heard about the book: "wood and canvas kayak building"
by George Putz (& if not, well, them, now you did).

building a foldable kayak means building any sof you want
in a "foldable way".

good luck,

post pics as you go,

Erez