Thanks for all the construction ideas - haven't settled on the final, non-cardboard approach.
But the cardboard internal structure is done. Finished the final bulkhead this evening, will start cutting, piecing and taping on the outer load-bearing skin tomorrow. It's supposed to be really nice out, so I'll be taking advantage of the nice weather.
Then it's mounting the tail lights & number plate on the transom, and after that, tuft testing.
I'll be incorporating a camera "boom" into the cardboard structure as well.