This is a guide to ensure fins are aligned properly on round model rocket body tubes. I've included the original FreeCAD design file so you can adjust sizes for various body tube diameters and fin thicknesses. As designed, the fin slot is 3mm. The fin should be snug, but not to the point where it leaves an imprint.

See more at https://youtu.be/soAqW4vl2hE

Update Jan 8, 2022: New FreeCAD file. Now fully parameterized to adjust sizes as required. Update the values in the spreadsheet and the model will be updated accordingly.