Filleting the joints
During the process of preparing the epoxy filler, I put another layer on the inside of the daggerboard box and the rudder holder. Then I thickened the epoxy with glass powder and some home made wood powder (from the sanding machine). Just thin enough for the syringe. Filling the room between the planks was easy, but with the last 1/4 to do, I had to make another cup of epoxy. After doing that, the syringe stuck because of the curing epoxy. I did the rest with a filler knife.
After one hour, I tried to smoothen the epoxy with a gloved finger that I make wet with denatured alcohol, but that was not easy. Final smoothening must be done with sanding paper later on.
One little problem: the room between the spacers of the daggerboard case is too small. I will have to make extra room with a saw in the inside. Only 2-3 mm. Or I have to make the daggerboard a bit smaller.