Parametric lens cap version of my bokeh stencil holder.
Just fill in the filter thread diameter in the scad file, adjust the paramers for the stencil card and generate a holder fitting your lens!
Additionally I included a simple generator for 3d printable stencil cards if you do not have a lasercutter. For more advanced, lasercut stencils check the original.
The stl file is an example for the Canon 50mm 1.8 II with 52mm filter thread.