A 3D model of a fin alignment guide style I've seen but not as a 3D printable model. Designed this jig to be used for low powered 18 mm model rockets, suits A8-3 engines and C6-5, and would fit any others that are 18 mm in diameter. This jig could easily be adapted to suit other sized rockets, feel free to do so!

I'd recommend printing 2 so that one can be used as a stand and the other a guide for attaching your fins

I like to create my body tubes out of Water-Activated Tape, wrapped around a 0.7 inch wooden dowel. This is just the right size for a body tube that fits the rocket engines. From there, insert your rocket body into the center of the guide, insert your fins at whatever spacing suits your needs and glue them in place!