Popular Science had a set of plans that looked very similar, I'll see if I can find them.

: I built it and it came out at 24 lbs. If I had put lightening holes
: in it I could have shaved AT LEAST 2 lbs and I suspect more. I
: think a 20 lb unit should be no problem. I used oak because I
: had plenty and it is strong. I think Baltic birch plywood would
: work and probably shave off a pound or more. Not that it 3-4 lbs
: is going to do anything but make it easier to handle.

: I have to make some changes, I found the outriggers are just to low
: and I couldn't consistently keep the paddles above the water so
: I have to modify it some. Once I get it dialed in I may make a
: new one just to see what I can get it down to weight wise. Once
: I get it right I will am thinking of creating a set of plans for
: it.