I used a solid core for the fins on mine made up of 3/16" x 3/4", wedge shaped, trailing edge balsa. Most well stocked hobby shops will have it in their Midwest displays. I glued the cardstock skin directly to the balsa strip, lining up the trailing edge of the skin to the widest part of the balsa. Then, cut the fin away from the balsa strip at the root edge, using the fin skin as your guide. You'll have about 1/8" inside the leading edge that isn't filled or supported but the cardstock is plenty strong enough in that area. You'll have to fill that tiny bit on the outer edge of the fin with Fill N Finish or cut a small part of the balsa wedge that came with the kit. One thing I've noticed about making these built up fins is that it's difficult to avoid letting the frame show through the skin once the glue shrinks. Clamping down while the glue is drying makes things worse. It takes practice. A medium or slow gel CA works pretty well since it doesn't shrink as much as white glue might. Or, use the solid balsa core and have a nice smooth fin in about half the time.
I have to believe the solid core is stronger than the built up fins, but this model is so light , I doubt it makes much difference.
