Hi fla and welcome to the forum.
I have three sailing rudders, each now using the winglet -- am quite pleased with their performance.
There are some things the winglet will do for you and some things it won't. It increases the authority and sensitivity of the rudder, especially at higher speeds. Thus, when sailing the AI, you can trim the boat with less rudder deflection and this has a slight positive effect on speed. It also seems to help some in tacking. Finally, it provides better broach control and stability during wake rides, where speeds often get up into the 8 to 9 MPH regions. It should also help control broaching in downwind sailing with swells.
IMO, it does not lift the bow, nor is the rudder system designed to support vertical loads. To the extent the bow lifts, it is the hydrodynamic result of the higher speed. This can also be accomplished with a weight shift aft. Any appreciable departure from 0 degrees for the winglet adds additional drag and increases stress on the rudder system and the probability of shearing the rudder pin.
My winglets do not help keep the rudder locked, nor do they hinder it. With the winglet, it's not hard to unlock the rudder. But with some speed, the winglet will keep the rudder from rotating up and out of the water. There are no miracles here.
I obviously try to keep the winglet close to 0 degrees AOA. When not turning, you want minimum drag. A couple of degrees variation doesn't seem to hurt and may be hard to avoid. Better to point down slightly than up slightly, as the water flow past the winglet may actually be rising slightly.
I guessed at the original shape and size based on what I've seen and what little I know about aerodynamics. My objective was to increase rudder authority in the pre-stall realm while minimizing drag (and snags) -- that's what came out.
I accidentally made the second and third winglets 1/8" narrower than the first, but was too lazy to start over. I also made them 1/4" longer (on purpose) to see if the change would "help" lock the rudder down. When all was said and done, they seemed to work about the same. I like your idea of trying something a little wider -- you might get better control in the higher deflection zones.
Some people have made a little deck cushion for the stowed winglet. I haven't done so but have had no problems so far.
Finally, to maximize performance I blunt the leading edge of the rudder to make it more elliptical, sand the rudder, taking out some of the warp on the trailing edge to neutralize any directional tendencies, and finish it off with about 400 wet dry paper. Take care not to round the trailing edge.
If you're familiar with the feel of the (new) standard sailing rudder, I think you'll notice the difference. Good luck with the project!