If you're open to modifying a kit (and your OP indicated you are) I'll tell you what worked for me. I reworked a Big Daddy to take a CTI 3 grain 29mm. I had to cut the bottom out of the nose cone (and reinforce its edge) to make room for the 'chute above the motor mount, but it all worked. New hand cut basswood fins, custom plywood centering rings, and a baffle build in to the forward end of the motor mount tube, a la Aerotech's Initiator (and probably other Aerotech kits.) It needed a bunch of nose weight, but in the end it flew beautifully.
