I built one when I was twelve. I modified the design by adding a steerable kort nozzle and an enclosed wheelhouse- looked a little bit like a cross between a Garnock and a Joffre.
I also made it plank on frame using plywood frames and pine strips- needless to say, I needed no additional ballast.
It wasn't a masterpiece, but being so small and agile it used to win all the pots at the steering regattas. Unfortunately it got sunk by a larger boat, and got so waterlogged, I broke it up.
May build another, as I still have the plan. Think I would use foam and build it in bread and butter, then skin it with light glass cloth. With lipos and miniature radio, the small size is no longer an issue.
Andy