Wayne,
We would be interested in pictures of your work in progress.
I do not have those plans which I guess are available from CKayaker?
May be I'll use ash-tree stringers and okoumé plywood stations.
As for fittings: heavy-duty aluminium horseshoe hinges can be ordered from manufacturers like Klepper, or trimmed out of wood. Others can be made of brass L or U dept-store fittings.
As for the skin: it seems you have tried those blue or green woven, thin plastic
car tarps. They just make good trial sails, not hulls as another post told you.
Truck tarp is a common choice. The wearing points can be strip-reinforced if you pick the thinner thickness quality. Stretch must be avoided. As home builders we won't snug and fit the skin with inflating sponsons, I guess.
It is available in white and in many colours.
One Ocean (ooops! competitor)has a page about Abrasion Resistant Fabrics, off topic I believe, designed for glassing.
Please see sites:

The Folding Baidarka Home Page
A New Frame by Thomas Theisinger (Pouch Boats)
Faltbootbastelseiten
Foldingkayaks.org
PaddleWise.net

Eric