I checked, and it looks to me like they changed the design. The current oen shown is plank on frame. if you seal the inside with epoxy you can cover most of the interior to seal it up. I honestly don't think you would have a problem

this one has no plastic inner core it is the 1/10 scale and all wood . throughout , it has a length of 3ft . 950mm
i know they have the different size kits

Given that the inner core of the riva is some sort of plastic (onto which the planks are laid and glued), and given that the one shown has some 15 coats of clear epoxy and varnish, it is quite well sealed. And stable in our experience.
I should have mentioned that you may be carving out space for the electronics and motors as well. (I say "may" because it has been a few years, and my memory is not photographic...)

i am very tempted by the Riva kit , but wondering about the wood issue with storage and shrinkage , tough call but such a beautiful boat

The Riva can turn out very well. My father and I built one. Personally I think it is easier as as static model. The hull is heavy and we had to hollow it out quite a bit to lighten it. We had a hard time finding figures that would fit it, as it is hard to find figures in that scale. Still, it WAS pretty when it was done...