I created 3 (so far) planetary gearsets, this one happens to be a 4:1 reduction, but I have made others that have different ratios. All of the planetary sets I made are compatible with each other and rely on a 0.500 inch or 12 mm hex as the input and output shaft. This means all of my gearboxes are stackable and can create different combinations. Each .stl file contains a sun gear, a ring gear, 5 planet gears, a carrier ring, and endcaps the enclose the gears. The holes in the corners allow for a number 8 or M4 bolt and nut to hold everything together.
Adjust first layer z to avoid too low of a first layer. Squishing the first layer too much will cause a flange on all the gears and prevent good meshing of the gearbox.