This post is very interesting to me as I was looking at doing something very similar about 2 years ago. I got sidetracked for personal reasons, but recently have restarted the build of my kayak. I was doing internet searches to help gauge interest in prefabricated components of kayak kits when I came across this posting. My approach was very similar to yours, I started with the Sea Cruiser 17R, parametrically modeled it, and even experimented with the best sizing for the tubing holes (something considerably under .75 gives a good tight fit in the material I'm using).

Where my approach differed was in my method of cutting out the components. I have played with 3D printing before, but to me, it is best suited for one-off prototyping. We also had some issues with the fusing between layers not always being complete. I cut out my parts on a 3-axis CNC machine. This worked very well, and I am able to hold tolerances of +/- .005." The downside is that my initial material cost is higher, and I'm sure I have more waste. If I were to market these parts, I'd have to charge more like $200 - $250 for a set. I use 1/2" thick Starboard, which is a UHMW marine grade (UV resistant) plastic. I have no doubt about it's strength and durability for this application.

Here's a picture of my frames cut out. They weigh just under 4 pounds or about 1.7 kg. I also cut out the coaming and the keel.

I will post more pictures when I get it on the strongback.

Kevin