This is a 20 slots speed encoder wheel with 38 mm outer diameter which fits tightly onto a 5 mm diameter shaft, such as a stepper motor. It has a thickness of 2.5 mm.
I designed it specifically to add optical speed encoder functionality when used with a double shaft NEMA 17 stepper motor. It was tested and works fine.
You can choose a lower infill setting but the wheel needs to be sturdy so that it doesn't bend and stays vertically stable without flexing too much as the motor rotates, especially at high speeds.
P.S. If you prefer to buy a more professional version, search for "M4-6-1 wheel encoder" on aliexpress. It is sold in pair as metal discs, each with 5 mm internal diameter and 100 slots which allows higher resolution.