In our studio, we have a shop-vac that didn't have a cap on it's filter anymore. I designed this to press into our filter so that we don't risk damaging the motor and are no longer spewing debris out the back. I made this part on OnShape; if you find yourself needing to change any dimensions, the relevant dimensions are kept as variables to be easy to change. 

My original has a 150mm ID filter, 20mm filter thickness, and a 2.5mm offset for a snug fit.

I printed one version with zero offset, and I had to add a lip of tape around the inside for a snug fit. I added the offset so that you could change how much interference with the filter you wanted to squish your cap into place.