When you just fill in a small gap on the edge, there is not enough contact area between the repair and the board. It is not as strong as the original so it is going to pop out or crush when it gets hit. If you want to do this I suggest Loctite(Hysol) or 3M premixed epoxy (http://www.scottsales.com/structural_adhesives.html, Look at the table or call the company to see which one is best) This is a lot easier than mixing it yourself. You can also try filled poly carbonate or urethane. They are more flexible and deform rather than pop out or crush.

The ugly truth is if you want a small repair like this to hold up, you have to use techniques you would use for a big hole in your hull. Grind a ~8 to 1 taper back from the damage (This area is so thin, you can probably only taper one side) Clean the area thoroughly with isopropyl alcohol. Apply a thin coat of epoxy directly to the repair. When that epoxy starts to kick, apply fabric or fibers with more epoxy to fill the damage and the taper(You want 70% fiber/30% epoxy by volume). Then use peel ply and plastic film and apply pressure (vacuum bag preferably or wood blocks and clamps). When it hardens, sand it smooth and apply a thin coat of epoxy to fill in any hole/gaps/etc. Finally the final sanding.

If you have room, bonding some very very thin nylon webbing to the back of the dagger board trunk with 3M 5200, helps a lot.