I heard that it happens when the nozzle is inside the BT by more than 1 tube diamiter. The nozzle would be inside by only about 7-10 mm.If the nozzle is up inside the boattail it will reduce thrust because of something called Krushnic Effect. Even if it's not in contact with the flame.
“Krushnic Effect: A very dramatic phenomenon where your rocket makes a tremendous amount of noise and smoke but doesn’t go anywhere! This happens when the motor is recessed into the body tube by more than one tube diameter. If so recessed, the cylindrical volume below the motor forms a secondary expansion chamber which allows the exhaust gasses to expand below atmospheric pressure before leaving the rocket. Surrounding air aspirated into the exhaust stream causes turbulence which negates much of the thrust, along with creating the characteristic roar. A multi-stage model that ejects its booster motor, but not the airframe, is a perfect example. Very damaging; it almost always destroys the lower body tube beyond use. Named for Richard Krushnic, the rocketeer who characterized the effect in the late ’60s. “
I'm not modying the organal closure but making a new one. So I don't know if that would be legal for a record.And the closure is "custom" but not modified? Anything other than the exact part from CTI would not be legal for a record. And that part can't be modified in any way, otherwise it's an EX motor, and not legal for records.
I heard it can also be done with 3D printed molds.Many people make male molds from styrofoam and dissolve it with gasoline.