Have just built a 1/4" ply boat, car top-able, light that planes with a 3.3 Mariner.
Have not weighed it yet, but would guess 30-35KG.
I found this old video
Not sure there was much epoxy in use then.
I have just made another boat to use for Seagull racing.
Stitch and glue, using 1/4" exterior ply and polyester resin.
I built it to...