Do you need to replace a fin from your Estes Doorknob kit or need to scratch build your own? I have the STLs for you to download for free from Thingiverse. You can print the fins flat or with the airfoil and with or without the tab. I also have a couple of fin making templates in order to scratch build your own from balsa, basswood, or other material.

https://www.thingiverse.com/thing:6339772

You can now download for free the STL 3D model of the nose cone for the Estes Doorknob kit to replace a damaged one or for scratch-building your own. There are two versions to fit 3" PSII tubing and 3.1" LOC/Precision tubing.

https://www.thingiverse.com/thing:6384202