Appreciate the replies Frank and Guy. After reading your posts, I've come around in my way of thinking on this, and think I may try the tape thrust ring approach (Method 2) to get positive forward retention. Per Frank's comment, using 4-6 wraps of appropriate width tape still renders the engine hook usable while providing an adequate thrust ring. And an additional "insurance" wrap or 2 around the motor and thrust ring may be all that's necessary to assure positive aft retention with the hook during ejection. (All without having to revert to traditional friction fitting of the motor into the motor tube).
I only intend to ever fly this Hi-Flyer XL with cardboard casing 70mm BP motors (or possibly with 70mm AP motors). This approach would allow me to continue flying on either D12s or C11's or E15s or E20s or E30s (or E9's / E12's if I can muster up the courage to fly these again after this last E9-6 CATO), without precluding the use of the longer 95mm BP motors. If things don't work out as planned, I can still go the route of gluing-in a new forward engine block, and restricting the rocket's future use to 70mm motors (Method 1), but I think I'll give the tape thrust ring idea a try first.
Thanks for the thoughtful posts guys. I'll let you know how this works out. (Or hopefully you'll see it yourselves at an upcoming CMASS or MMMSC "big field" launch.)
