my least favorite part of building a rocket is the motor mount. It requires a handful of parts, a decent epoxy system, and is one of the more expensive parts of the airframe. I've designed a modular mount that i can pull out of one airframe and plug into another. So once the motor mount is done i can use it in all my airframes of the same diameter and motor diameter without any bonding. To cover all my current airframe diameters and motor diameter combinations requires 5 prints and adding a new one is pretty easy.

my next thought is modular fin cans. The rule of thumb for trapezoidal fin dimensions are a function of airframe diameter. I would like to be able to make one fin can for each airframe diameter and then re-use those on new airframes. With my current inventory I need 4 fin cans ( 2", 3", 4" and 7.5" diameter airframes )

Not gp, but things you can definitely use 3d prints for related to rocketry: nose cones, fins, motor mounts, electronics mounts, and probably a whole slew of other stuff I'm forgetting at the moment.