Solid knock box due to using a piece of 20mm electrical conduit covered with a flex, you could replace the electrical conduit with a printed tube 20mm diameter and 115mm long. This knock box has stood a year of abuse and still going strong.

I printed the box in ASA 15% infill but I think PET would also be OK

I used 95A flex for the tube but any flex that can soften the hit with a portafilter will be OK.

Conduit with Flex cover.

Conduit 115mm long

Conduit 20mm Diameter

I also glued some nonslip material to the bottom to make it stable on the bench