I've been doing a lot of CFD analysis of T foils, centreboard and rudder interactions and on planing hull trim. So here is a quick summary of findings
1) T foil sections
=============
Best to use asymmetric section with about 10% t/c. Speer H105 is a good section because it gives low drag a zero lift and has an extended drag bucket at higher lifts.
2) T foil planforms
===============
Assuming you don't want a span >900mm, use an approx. ellipse. You can get approx elliptical loading by having a trapezoid with a taper ratio (tip/root) of around 0.5. Make tips rounded.
3) Position on rudder
================
The T foil lift is of no benefit below about 6kts and so should be a zero lift below 6kts boat speed - see Y&Y article for reason.
If your boat is in bow down trim at say 7kts, best place is as close to the water surface as possible to take advantage of the upward flow off the bottom of the hull. Most cherubs are too fat in the bows to do this and start to adopt a bow up trim. One this happens, the near surface advantage is lost.
My thoughts are that it is more practical and easier to fix T if it is at bottom - makes launch and recover easier. See also 4)
4) Aspect ratio of rudder blade
========================
Given the normal ratio of rudder to CB sizes, the foils will give best lift to drag when the rudder is unloaded (i.e. practically neutral helm when boat is upright).
The trend on I14s for long skinny rudders makes no sense if the rudder is only lightly loaded. This is because only the induced drag (the drag caused directly by producing lift) is reduced by aspect ratio. If the rudder is lightly loaded, then the induced drag is very small.
For control at low speeds, pre start moves, and general control, you can do for a wider chord (25 cm or so) and a short blade. If you stick the T on this, it will be pretty near the surface anyway.
5) Using a T foil
=============
For any given speed, a hull will have an optimum fore and aft trim angle that will give least drag. It is important that this is maintained when using the T foil rudder. When at planing speeds, too much trim will be less detrimental to speed than too little, so don't pull the T on so that the bow is in the water. You must be able to place your weight sufficiently far aft to maintain the same optimum trim as you would without the T.
6) Notes on optimum hull trim
========================
When a boat is planing, the optimum trim angle depends upon the lift coeff of the hull's bottom. If your boat has a fair bit of chine rise in the back, trim will be around 7-8 degrees . For a real flatty like the Dog, it will be nearer 5 degrees.
Zero lift on a symmetric foil is at zero to the water flow. So on the Dog this would be at about -5 degrees to the boat. Zero lift for an asym foil is about -3 degrees, so this would be a -8 degrees on the Dog.
In practice, you will never use a T foil a negative lift.
7) Notes on nosediving
==================
If your boat is nosediving or sailing too bow down with the T foil fitted compared to when it is not fitted, then the foil is producing lift.
The balance of a Cherub offwind is a precarious one because of the ratio of rig height to boat length. The C of E of the rig with the kite is about 45% of the way up the mast. This produces a moment trying to pitch the bow down. You try to counter this by moving aft.
If the drive from the rig (with kite) was horizontal, practically any Cherub would become unsailable as the wind increased. Fortunately, the drive is not horizontal. If you have a long pole and a highly raked mast like on Atum the drive is upwards at about 34 degrees to the horizontal. This reduces the pitch down moment and lightens the boat, as does the T foil. (See WB sails website article on Lifting the bows of a Cherub)
If your boat is really divey, then you need to increase the rake, increase the pole length and lengthen the wheelie bars. If you imagine the line of the drive vector from the rig and extend this line backwards, it will eventually cross the line of the boat at sheer level. If, where it crosses, is in front of where you can stand (lots of rake and long pole), then the boat will never dive. If it's well behind you (short pole, upright mast) you'll be sailing a sub.
I hope these comments help.