If you build model rockets you will probably know that aligning the fins on your rocket can be a fiddly task. From measuring up the tube, to mounting the fins at right angles to the body, mistakes can lead to a rocket that does not fly quite true. I looked at the commercially available jig from Estes as well as some other DIY solutions. However I wanted something simple to use that was reasonably scalable. This is what I came up with.... The tools is a jig that inserts over the body tube of the rocket. It is parametric so you can change the number of fins, tube size, fin width etc. to suit. The tool holds your fins perfectly true whilst glue sets on the model rocket.Measure your rocket parts in particular: Add the measured dimensions to the openscad model and adjust the jig height, wall width and the gap width (for the root of the fin to prevent glue bonding the jig to the model). Print the part, clean up any flash and insert over the end of your rocket tube. Insert your fins with the root edge glued. Wait!