Plywood would be cheaper, lighter, and easier to build.
Like the fellow wrote "stitch and glue", also called "stitch and tape".
A cheaper building method is chine batten construction which uses strips
of wood along the seams instead of resin putty.

The foam boatbuilding I've seen described requires a building frame to which
a special boatbuilder's foam is attached. It's then covered on the outside
with lots of fibreglass for strength. Then it's femoved from the bulding
frame, turned over, and covered on the inside with more fibreglass. All
that fibreglass makes it heavy.
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