Very nice! What do we think the optimum span of the T should be? as i'll be cutting down the prototype Atum Bom uber wide special soon.
I suggest there are 2 factors you need to consider as well as transport.
The first is the area of the foil required. The max L:D of the foil occurs at around a CL of 0.5 (5 deg for a symmetric foil). You can calc the lift from 0.5*Cl*1000*V^2 where V is the boat speed in m/s. To get say 70kg of lift at around 8.5 kts I think I recall that sum gives you about 0.1m^2. The windsurfer foils Will stuck on the bottom of the rudder are way bigger than that which would explain why they were draggy, so I suggest you chop some off the ends to give an area of about 0.1.
The second thing is that for a given area, increasing length (AR) lowers the induced drag. But if you place the foils 1/2 way down the foils like Phil to gain lift from the hull's wash, then there is no point in going wider than 900mm. The reason for this is that the zone of water affected by the hull is roughly a semi circle of 1/2 chine width radius extending below the hull.
Finally, to clarify a point often made, the thickness of the foil doesn't affect its lift for a given angle of attack. Thick foils can operate at higher lift coefficients than thin ones, but have a lower max L:D. You don't need really high lift coeff on a rudder T, so a thickness to chord of 10% is a good compromise. Laminar flow sections are also good. These tend to be a bit sharper at the front and have the thickest point at around 40% back.