wow, what a great idea!
Do you think the structure is sting enough?
What do you think about epoxy inside?
First I plan to do the structure with wood and outside I use some layers with carbon and epoxy.
But now perhaps I print the parts like you with my Anycubic Chiron. Less infill and epoxy and after I make 1 or 2 layers with carbon 
I’m building a box at the moment. Printed parts with low infill, glued together and then laminated with fiberglass. Bottom with alu plate 4mm. Top with carbon sandwich. Parts are printed.
I’m quite sure you don’t need any extra reinforcement. With either epoxy or polyester, the frame becomes rock solid. I’ve tested booth and my reason for using polyester is the price. Instead of just glue the parts together like @sat_be suggests, cut off the solid walls between the joints and then, when you pour resin inside you will have an almost solid part. I’ll explain more in my coming post later this weekend
So, after around two month i continue my work in CAD and did some testprints. PEr example if my Mastclamp fits to my 63100 motor.
Here a pic of all what i do.
and my testprint
Inside my Box i have a lot of work to construckt. I will seperate it in 3 areas. (Batteries, Elextronic and on save closed box that stops when water coming inside my mastconnector)
… and here the other side on my technik box.
… here my mastclamp with duct.
The duct i decide to mount on the Fuselage. After printing in 3D i will coat it with CFK. I hope it is strong enough, when not i can easy change it with a more stronger model.
So, I bought the Flipsky 120kV Motor and change my CAD constructions to that system.
Here is my Fusion 360 CAD.
I decide to print first the Duct in PLA.
I want use the same colorscheme from my Gong Foil. I think the colors are very similar 
Very interesting. We could imagine that the front section of this square shape could have the same section as one half NACA airfoil section to minimize drag.
You do not need a grid at the downstream side, only upstream is important, because the propeller will repell anything which comes from the backside without danger. Only if you plan to spin the motor backwards it makes sense to have a grid at the back. Also the grid at the front side is too wide to protect the feet properly.
At moment I print my Duct without a grid!
The grid I want only when my children riding my efoil. I am working for some details for save my children. So it should not possible to start the Motor when a magnet is mount over an Reedswitch.
The grid should me only that nobody push the hands into the Propeller from the back. You are right, the front part is not save but I hope some one not put the hands complicate from the front to the running Propeller
So, the next part is printed.
I calculated the Prob Online with that link.
After theme I downloaded the Cad File and import it in my Fusion 360 and made mu constructions.
The result is a nice Propeller that fit exactly on the shaft from my motor.
Wait, you are printing without supports and still getting that quality ?
I would 100% fail I think
Yes, but on the underside i have not so a nice surface. But absolute no problem because I can clean that with a file.
Can I ask how much did you pay for this board ?
It seems a good shape and hole looks great too !
It was a prototyp, i payed 400.- for it.
The company is a Startup, based in CZ.
The boards for selling from theme, made in Carbon costs around 800.-
I also use the Anycubic Chiron. It’s a good printer. But I made a few modifications, which improved the print image even more.
On your propeller I also see potential for quality optimization with the right settings.
Overhangs up to 65° are possible without support.
The blades look thin, I suspect they will break. I have had a good experience with my impellers with a blade thickness of 3.5mm and PLA pro.
PLA pro can be post annealed in the oven. It is then harder than ABS. And very easy to print.
