Kevin, that looks really good. If I were you, I'd just go for it.
By saying "go for it", I mean skip building the tube and coroplast prototype, and build the final version.
To get those curves to work out you need slices like orange slices. I'd get the dimensions figured out, and then cut plywood stations and attach them to a keel to keep everything lined up. It is most like a boat hull and that would be easiest; why reinvent the wheel?
The beauty of curves is they become rigid all by themselves without the use of any framework. If you can keep curves in both planes it will be very strong.
Build the bulkhead stations and keel and get them set up about a foot off the ground and straight on a framework. Start cutting your orange slices from long cardboard if you can find it for free/cheap. Fit them with tape.
Get the shell covered on one side. Once you have it looking right, take it back apart and use the cardboard slices for a template to cut your final material. you should be able to make two of each, one for left and right.
Now you can choose coroplast for a core if you wish, or some other material. The strength will come from the epoxy and glass you will put over it, so the coroplast would work fine.
I'm suggesting a method like stitch and glue for lightweight boat building. You could use Okoume plywood and give it a bright finish if you like wood ( I do! )
Coroplast is 5 lbs/sheet for 4MM, $17.50 per sheet. It would do fine and be a good base for a painted shell.
Okoume is 15 lbs/sheet for 4MM, closer to $50~60. Far stronger and you'd be tempted to leave it natural.
Here is the stitch and glue method I'm suggesting.
Stitch-N-Glue
That 20' long triple cockpit boats weighs in at 64 pounds
The final product will still appear to have slices and slight corners but it will mimic the curves very well.
You can choose to attach to your bulkheads permanently, and then use a sabre saw to cut almost all of it away, leaving a 2" bulkhead ring for example.