The thin acetate fins that come on Stomp Rockets quickly fell off ours. Found these made by Matchstick as a great starting point:
https://www.thingiverse.com/thing:2946897

I actually usually shorten up the Z axis in Cura to about 30mm instead of the 40mm. Still flies great, but less time to print. Turn off "Uniform Scaling", just shorten Z, otherwise won't fit on your rocket tube.

I might thicken up the fins a little for durability, but after many flights, only one fin has a small crack out of the bottom corner, still flies. Not very durable, but quick to print and lightweight.

The small hole is for a dot of super glue, keeps fins from sliding up or down. They fit on pretty tight, but an adult launch was able get fins to fall off the bottom. When they land nose first, they slide up. A dot of CA stops that in both directions.