The other way to use extrusions could be as a mould for the mast. After moulding the mast inside it, grind away each end of the extrusion and peel off the aluminium skins. You would then have to mould on the nose and tail of the mast.
They would be perfect and not need modification for bi-directional.
There have been studies on pure ellipses and I'm not sure if any of the extrusions are pure ellipses, but this study seemed to find that somewhere between 12% and 16% thickness/chord ratio seemed to give the best bi-directional performance.
Page 82 illustrates ellipses with 5%, 10%, 16%, 20% and 25% t/c ratios.
Page 80 shows 12% to have the best lift/drag ratio but in the diagram above that, it shows that 10% is prone to sudden stall, whereas 16% has smooth lift/drag as the angle of attack changes.
Its a pity they didn't do more studies between 12% and 16% t/c to find the best compromise.https://mospace.umsystem.edu/xmlui/bits ... sequence=1