Some thoughts from a man who has never attempted any of the following.
You may want to put in a solid piece of wood to mount your boiler first, doing it first it should be more secure. It may take a bit of shaping but will be worthwhile.
The second boat appears to have an inner wall of timbering, as if pannels had been laid over the ribs of the frame, creating a double wall effect.
If on a real boat I imagine this would be achieved by planking horizontally from the handrail down, or from the deck up. to achieve this you would need some kind of spacer between inner and outer wall. Mock ribs would help you achieve this. You could then plank over them horizontally, or use sheets of very thin ply (0.4mm) to follow the curves.
The canvas effect might just be cotton sheet, laid on the the inside of the boat, a water resitant PVA could do this.
Then add your wooden floor/deck.
Then the wooden strips/frames could have been laid over the cotton, and not indivual cotton pannels between frames. And also along the edge where the wall meets the deck.
If you are not after a double walled effect you could try this.
Get very thin balsa planks (perhaps 0.5mm) and glue horizontally to the inside of the FG hull, this would simulate the hull planking as would be seen from the inside on a full size boat. After gluing with CA, a few coats of varnish and they will be rock solid.
You may want to put in a solid piece of wood to mount your boiler first, doing it first it should be more secure. It may take a bit of shaping but will be worthwhile.
You should also add mock ribs. You will need to think about where they would have been sited on a boat of this scale. Again this could be achieved with balsa. Balsa I believe can be made to bend like rubber with a 10% solution of ammonia or ammonia based cleaning products. You can use heavy square section balsa or you could laminate several thinner more flexible stirps.
Finally lay in your floor.
For some reason I do not think the internal planking would be vertical. Possibly because of curvature, and possibly because all those cuts require more labour in a real boat. But what do I know? ::)