All the solutions so far have been targeted at stopping the symptom (water coming out th top of the trunk) rather than the cause (which is not a leaky top of the trunk). Everybody's been proposing ways to fix the top of the keel trunk -- but I think they're looking at the wrong end of the keel!!!
If I treated my patients like that, I'd either be out of business or very rich from all the office visits! You have to diagnose the problem, not just focus on the symptoms. Some neck pain is caused by bad biomechanics in the neck -- but sometimes neck pain is caused by a distant ankle that is total posture. You gotta look for what's not working right.
Okay, the water coming out the top of the trunk is a pain in the neck but is it caused by the design at the top of the keel trunk? I don't think so.
The problem is where the water GETS IN, not where it comes out. The opening at the bottom of the hull for the keel is pretty big compared to the space and the top. The pounding of the hull on waves and the pressure created by the movement of the hull through the bottom is pushing water into a large space, forcing it up, and creating enough pressure at the top (where the opening is constricted) to force it out a pretty well fitted, but not watertight joint.
So, to correct the cause of the problem, you need to reduce the size of the gap between the the keel and the trunk at the bottom -- if you can make it smaller, there will be lower pressure at the top where it leaks into the boat.
I'd suggest taking some indoor/outdoor carpet and glueing it inside the keel trunk at the bottom to fill the gap around the keel. If very little water gets in, there will be no increased pressure at the top, and the top won't leak. The keel will slide easily past the carpet, and reduce keel wiggle to boot!!
I did exactly that on my 14-foot Force 5's and they no longer leak at the top of the trunk. The builder sells a very substantial piece of neoprene cut to precisely the shape of the daggerboard, but after about 30 days of sailing, it inevitably gets a small rip or two and the water spouts up through the rip. I'll bet they make a lot of money off folks who keep replacing the gaskets...
I've treated many a "pain in the neck" by treating the ankle, knee or hip. You gotta examine the patient before you can get a good diagnosis and fix the problem. Take a look at the whole keel trunk, not just the top and see if this would apply to your boat.
Just my $0.02, but I think it's worth alot more than that.
Dr. Judy B