Small motor airbrake

Did this quick airbrake to conduct rmp measurements on small, electrical motors, using this cheap tachometer:

http://www.banggood.com/Wholesale-DT2234C-Digital-Laser-RPM-Tachometer-Non-Contact-Measurement-Tool-p-20463.html

Current design is for a 3mm shaft.

WARNING spinning printed objects at high RPM could lead them to violently desintegrate, wear eye protection at the least.