Update 7/2: v2.0 - Now with sleeve option for attaching fins on different places along the tube.
I wanted to make it easy to attach just about any number of fins on just about any size rocket tube, and so I came up with this fully parametric model. You can even print one that supports different numbers of fins at the same time (e.g. 3 and 4 -- see photo).
It's smart about fin alignment, too, providing internal support where the fin is attached, and offsetting the guide block to compensate for the thickness of the fin material itself.
I printed a rather small one here so we could fasten three cardboard fins onto a paper towel tube, but it's very easy to configure it for something different. For example, you can print one to attach five fins to a thick shipping tube -- just tweak the parameters a bit. I've annotated one of the images so you can see many (not all) the parameters that are available, or you can just look at the comments in the OpenSCAD code. There's even a standoff setting to help prevent gluing the jig to the rocket.
It's also smart about filament -- there's not a lot wasted here. The one in the photos required about a meter of 3mm filament, costing about US$0.25.
The STL is useful for gluing fins cut out of beverage container cardboard on to a paper towel tube (as you'll see in the pictures), but you'll probably want to configure it to suit your own specific building materials.
Available parameters define the size of the rocket body (inside and outside diameter of the tube); the number of fins; the fin thickness, height, length elevation, and standoff (from body); the thickness of the fin support material and spine length (for stability); the thickness of the support material for the tube, and the thickness of the base.
1) Change the values in "User-defined values" to suit your needs. If you want it to support both 3- and 4-fin configurations, use an array like [3,4] for finQty.
2) Print and clean up if necessary.
3) Insert tube, and clip fins to the guides. You can add glue before you attach them, but be sure you have set the standoff value so you don't accidentally glue the jig to the rocket. I just glue one side first with a bead of hot glue, and then remove the jog to glue the other side.