After some delays due to buying new gear I wanted to try rather than testing old stuff I got round to adding the mast fence. It made some improvement, the foil is now reasonable to ride, ventilation still occurs but only when the main wing is very close to the surface.
Top speed has now increased to around the 17 knot mark (biggish wing) but after riding a few production foil I know I just don't have the tech to build a mast stiff enough so the board never really feels really solid underfoot.
It's been fun building them but after a few years I'm over it, will flog the homebuilds off to learners to help them progress. Homebuilds work just fine, but to feel great when your a experienced foile, that's the hard bit. Build one for the enjoyment, not to save money.