I recently ran across this folding kayak
designed by Anton Willis and constructed from a single sheet of 12' x 5' corrugated plastic (aka coroplast or corflute).
I thought it looked really interesting considering he is using only folds to strengthen the kayak longitudinally. The guy who created it has a patent pending on the concept and I ran across his patent application
which included a layout.
Looks like an interesting project. Almost certainly not as seaworthy as the inflatable folder designs Tom has provided, but might be a fun flatwater boat for small waterbodies.