Hi, I recommend using a 25% inner support. Then you know the exact volume of the support and you can calculate the thickness of the shell using simple volume formulas. If it is too heavy, simply file some material off You will need a bit of trial and error if you want it to be very exact, unless you take into account all extrusion parameters, for example measuring the filament diameter at multiple places. Any FFF type 3D printer will do, make sure to print thin layers for a better waterproofing.