It worked out well, although the case may be a tiny bit too 'loose'. I'll probably have to make the plastic block at the back of the case 1 mm less wide, to squeeze the case a bit together.
Couldn't think of a good way to put the glass spacer plate in the center of the molded case, and just guesstimated when I glued it on. To glue the tube properly alligned to the spacer plate, I put the case upside down on the rudder and used a piece of tube as can be seen in the picture below. I'm sure this method is not very accuate, but it was the only way to allign the tube I could think of.
In the meantime I'll also work on the centerboard. I wanted to use western red cedar as a core, but changed my mind when I found out in the Netherlands this is not the 'economy option'. Divinycell is cheaper. I think it's quite a big blade; should be easier to shape and laminate than the rudder.