Years ago when I was 15, a couple of us worked with our scout leader to research and draw up plans for building our own kayaks. After considerable research (pre-internet so we did it the old fashioned way via libraries, books, personally talking with experts, etc), we created our own design for a single user kayak and paddle. The original design was created to allow either canvas or fiberglass kayaks.

The first ten kayaks were made by myself, our two leaders, and several other boys. We cut out and assembled the frames in our leader's backyard, then all worked on covering the first kayak with canvas, hand sewing the darts, sealing the canvas, etc. Each kayak was covered with one piece of canvas so there were no seams to worry about. We made them using canvas since I was able to get wholesale prices on tent canvas (uncle worked for a canvas ten manufacturer in Sacramento). The total cost for the wood, canvas, wood screws, heavy duty thread, wood sealant, canvas sealant, and paint was under $20 for a 16' kayak (1968 dollars).

We tested the kayaks on a small lake, then on the Columbia River. From there some of us did lake kayaking, white water, ocean kayaking, surf kayaking, duck hunting, etc. None of us ever had a problem with durability or safety, except for the kid who blew out the end of his kayak with a 12 gauge while duck hunting (gun safety issue not kayak problem).

Hundreds of kayaks were eventually made from those original plans, most using fiberglass since few had access to the wholesale canvas like the original group.

BTW, none of them looked like the hack jobs seen in the picture accompanying that story about the homemade kayaks.