A couple of techniques showcased here. The quickest, simplest method is to cut the bottom off of a one-liter soda bottle, as shown on the left. The bottle on the right has had the end heated a bit, which curls the plastic under and makes the bottle sturdier. In retrospect it's probably not necessary. It has also been sanded with a fine grit sandpaper to give a smooth luster. Also unneeded, but it looks kind cool, is easier to see at higher elevations, and glue tends to stick to it a little bit better.
Until just recently I alternated between using Shoo-Goo glue (for fixing sneakers) and silicon weather stripping type glue. The shoo-goo seemed to dry firm and solid, while the silicon was nice and flexible. The shoo-goo setup quicker, which is the main reason I used it. Still, with either method you really need to let the glue cure for a day or so.
And then I discovered hot glue. You don't want to use a glue gun on your main bottle since it can cause the plastic to weaken and blow up under pressure. But, since the one liter sleeve doesn't hold any pressure, we can glue gun with glee! You can make or repair a one-liter sleeve in a few minutes if you have pre-cut fins.
Fin material should be light and rigid. We had spare scraps of vinyl siding in the garage which seem to work nicely. Another good source is the plastic shells they sell small electronics items in. I usually rough up the edges of the fins with sandpaper, or even put small holes in them, before gluing that edge to the rocket. Anything to avoid having to glue them on again.
This is the jig I use for aligning and gluing the fins. Actually it's the bottom of our bird feeder and it just happens to have four evenly spaced holes and the one-liter bottle fits snugly over the indent. If you aren't as lucking in finding a ready-made jig I would suggest rigging up a small piece of CPVC vertically on your workbench and sliding the bottle down it to keep the bottle perfectly vertical.
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