There are a zillion vids on youtube like the following. I picked these two almost at random. The second one, where a guy builds a pontoon boat out of styrofoam, is how I've envisioned making a SUP board for a while now. If I did make a foam SUP board I would make it as wide as possible. I'd experiment with shapes until I found out what "too wide" is. And then I'd back off an inch or two.
The current SUP boards that sell for $1000 or so were inspired by surf boards. They're all too narrow and unstable.
https://www.youtube.com/watch?v=NPLk4q9K6rs&feature=youtu.be
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