Hi Duncan have you looked at Gupi's build log? He shows photos of his keel, no dimensions. It is not a method I would use too much engineering on show. However the area of the keel looks to be about 6" x 2" with about a pound of lead underneath, so that is useful to know.
Bearing in mind the internals are 10+ years old things have changed a bit now.
Gupi suggests 2 sail winches, yes but only 10 years ago, I think one internal winch will work. Howes do them, called Kingmax sail winch, need careful setting up I have one operating at least the same sail area in a yacht. The winch line is all inside the servo drum so no need for an out and back system. The ESC would have to have the red wire removed from going into the receiver. I use a short servo lead extension for this and simply cut the red wire, so no damage done.
The battery is a bit powerful for this boat but depends what you have to hand. I reckon the receiver and winch can operate off a 4.8 v 2900mAh AA battery. The motor drive also AA but 6 cells. Makes the disposition of weights and balance easier to adjust.
Could go for a 6 cell NiMh for everything, but they are bulky to decide where they will fit.
With the removeable keel I would rather create a small box in the hull and a slot in the keel to enter and then a close fitting pin to hold it in place. Only needs to be 2 inches inside the hull and waterproof, so only a minute to fix or remove.
I have in the past used old boxwood ruler as my keel wood source for a small yacht. It is very stiff and maybe glued edge to edge and some 1/16th. ply each side and sand to an aerofoil section. A stick of old fashion solder might be enough for the keel,
cut in 2 and screwed in place on the bottom of the keel, filing to shape as well.
Lots to think about, it is worth stream-lining everything under water as it is 800 times denser than air. (Purists would say 784).
I am thinking about disabling the drive motors when sailing, not sure how but depends how many channels I decide to use. This way the rudder can operate without bringing in the drive motors.
I find things evolve as you do them so this would be a start for me. I like things simple as my memory is a bit dodgy now! I write it all down in a note book which helps if there is an interruption in the building.