A key component of every rocket is its motor mount. The motor mount for my Level 3 rocket is designed to accept a 75mm motor (nominally the CTI M840), so its core is a 3-inch cardboard tube. I started by stripping the outer layer of the tube so that glue would adhere better to the tube. (Failure to remove the glossy surfaces of tubes has come back to bite me in the past.)
Because the motor mount is so long (36 inches) and because of the design of the fin locking mechanism, I elected to use four centering rings. The forward and aft centering rings are 1/2″ plywood and the inner two centering rings are 1/4″ plywood. The gallery below shows the slot-and-key pattern used to lock in the fins. These centering rings were first bonded with a few layers of yellow wood glue. Later, the joints were reinforced with JB Weld.
Originally, I had planned to reinforce the motor mount with three threaded rods (hence the large hole pattern). It turns out these were not structurally necessary and were adversely affecting the weight and weight balance of the rocket, so I cut them out. One positive thing that did come from the original installation of the threaded rods was a better alignment of the fin slots.
The forward centering ring has four small holes in case I ever wanted to put a cap and U-bolt on the booster section. This configuration is not used in the nominal construction of the rocket. The aft centering ring has six holes along its outer diameter and twelve along its inner diameter. The holes (and tee nuts) on the outer diameter are used to bolt in the motor/fin retainer assembly. The inner hole pattern exists to give clearance to the screws that attach the motor retainer to the fin retainer plate. Even though those screws have been cut down, small bits may protrude upwards, which could affect the ability of the retainer assembly to sit flush against the motor mount assembly. There is a 0.25″ overhang of the motor mount tube past the aft centering ring, which allows the retainer assembly (1/4″ plywood) to engage into the motor mount tube.