I actually found one of those plastic spin-fin units in the first months I was in the hobby. I found it on the beach after the 4th of July. It has split along one side, but seemed like a perfectly good thing for using to build a rocket. I rolled a tube using the instructions in the plan book that came with my starter set. Carved a cone out of a nubbin of wood.

It wasn't until years later that I realized that this item was sold by Estes back in the day.

It was cast in a slightly flexible silver plastic, similar to the plastic used in firework nose cones.

FYI, plastic fin cans are sold by cannonfuse.com. Two styles, neither quite like the Dirty Bird unit, but close enough.

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NAR #27085 - Oregon Rocketry - SAM