OK, think I'm far enough along in the build to post some pix and info. The first hurdle has been deciding what glue to use at the fillets where the balsa fins meet the plastic cone. But before that I did some fin prep by applying a thinned layer of Elmer's wood filler to each die cut fin, allowing it to dry and sanding it smooth.
The next issue is after cutting out the fin slot holes in the plastic cone is the fit of the fins themselves. With the adapter built but not glued in place you may find some gaps between where the fins and the cone meet. I filled these in on the fins by adding some thin balsa to the bottom edge of the fins where they meet the cone. This was a time consuming process because if I added too much then the ttw part of the fins would not meet the motor mount. One nice touch by Estes is that they actually allowed for the right "step" where one ttw part of the fin meets the engine clip retainer outside the motor mount.