Back when I still had hair (but before I got personally involved in all of this kayaking stuff) there weren't many commercially available boats and most people simply made their own. Clubs often had molds for the use of their members. Walbridge's book is the canonical reference. If you really intend to build and want a copy let me know, I may be able to find one. Although I've never built a complete boat, it is also a useful resource on repair and, with my boating skills, I've done a lot of that.

Frequent contributor Reg Lake was a whitewater pioneer and a is a first descent legend hereabouts, and he says the motto in those days was "Build 'em light and often."