Surfski rudder plug for moulding/casting from.

240mm deep, volume of 165mm. Skinnier than the carbon fibre think rudders.
Aluminium has a density of 2.7 x that of water so an aluminium rudder of these dimensions will be approx 280grams when submersed. To protect the surface you can wire brush, annodise and dye....google it.

The rudder has a tapered shaft entry, 8mm tapering to 4mm diameter over a length of 60mm. There's also an indentation for drilling and tapping a grub screw but it's better to cut some gripping notches into the SS ru