I did some real-time design and slapped this together. As expected, I went for an 18mm mount. The tube fits perfectly in base of the fin can. I thought that I might be able to ream the fin can out to fit a 24mm tube, but it is too thin. At least it's too close given my eye-hand coordination.
You can see the motor mount in the 2nd photo. An 18/24mm ring, shown unattached, will be mounted below the fin can. A 10" section of thick walled BT-50 slides into the Griz Missile's tube. The upper ring on the motor mount will be bonded in that tube and will hold everything together. I couldn't fit a motor clip without either having the motor mount sit well below the fin can or cutting a notch in it. A steel fishing leader and a little elastic will form the shock tether.
The nose is a Despicable Me 'mutated Minion' toy from McDonalds. I may replace it with something else but thought I'd see how it looked. I made a shoulder from a 24mm case and screwed to to the toy. The toy has a whistle protruding out the back. I'm going to find some tubing (maybe a bendable straw) to route the whistle inlet so it points up into the airstream. I don't know if it will make noise, but it is worth a try.
The last thing would be to make a standoff to hold a lug so the rod will clear the Minion's bulbous head. Since my next MDRA launch is a ways off, I won't add the standoff until I finalize the nose cone.
I weighed the major parts and made a quick RockSim model. So far, it looks like it could fly on a C6-3 but. I am also kicking around other crazy ideas like CHAD staging it or adding two 13mm boosters.