Not really applicable here imho. Most high power high performance rockets aren't cardboard. The majority are either fibreglass or carbon fibre.
Flex doesn't really become a concern with carbon fibre until you approach Mach 3. I say this because I've built two rockets of an identical design philosophy; one for 54mm motors and one for 75mm motors. The 54mm motor rocket did M2.64 and I could fly it again today if I wanted. The 75mm rekitted itself at M3. It failed where I'd not reinforced the hand rolled airframe that I was flying. 4 wraps of 2x2 200gsm twill CF. Same number I used on the 54mm fine.
For both of these rockets the vast majority of the airframe is directly over a aluminum motor casing that is a rigid body. These things are grossly over-engineered when compared to the pressures/forces they see.
