There is another option for core material I am using to rebuild my rudder. The original with three SS ribs and foam core with glass and Kevlar skin was totally waterloged. I built up inner structures and then filled with syntactic foam. The mix I used was 3 gallons of low viscocity Epoxy resin and 7 gallons of 3-M micro-balloons. The bleded gooy mass is like warm Kraft Marshmellow creme. Cured, it is supposed to have a compressive strength of 3000 psi. The resin is no-blush 635 from US Composites. (they also sell the balloons and cloth) The "foam" can be easily sanded and shaped. The finished skin will be graphite & vacuum bagged. My intent is to have a truely water-proof structure.
You might consider using this system to form your core in situ if you can leave the insided glass layer in place. (make gravity your friend). I have also found that you can form the stuff if you put down the foam and apply a release film. If need be, you can also press the film to hold it in place by using cardboard forms and props. You should get excellent bonding to this material as you build up the top layers. I suggest you try a few resin & balloon ratios to decide what will work best for you.