If you build high power rockets you know what this is useful for.
This is for 3" G10 fibreglass airframe, but could also be used for G12 (with a little shimming tape)
This is a 3" fin guide for 4 and 3 fin rockets. It also has corresponding alignment guides and vent and switchband drillhole guides.
your choise PLA or ABS, I suppose TPU could work
I also design custom sled applications; battery and video camera applications for model high power rocketry. Except for AV-Bay applications, I do not use 3D printed parts in HP rocketry as I feel the material is not yet optimized for this application. So most of my designs and builds are tools, jigs and holders.