Thank you so much ! I will keep you updated from my side !
Ok, I found the way to edit .stl files with Fusion 360. I added two mods to your "Classic" :
Printer setup is the same than the one you recommend (infill15% and perimeter = 3)
Hardware needed :
For the rudders: 2 rudders, 2 M2 x 13 screws, 2 M2 flat washers, 2 M2 nuts, 2 tiny droplets of thread lock
For the ballast : 4 M5 nuts
See picture 1 for the rudder installation.
I created 4 wells inside the hull in which I fitted 4 M5 nuts as ballast (see picture 2). The nuts must be inserted when the machine is printing layer #28. No need to pause the machine. I added a drop of cyano to secure the nuts, but I think this is overkill.
This design is yours, Keye, if you want to upload it. Thanks !