The bottom of my coffee knock box eventually cracked under repeated impact, so I designed a new housing featuring a TPU inner layer and a rigid PETG outer shell.

The TPU core effectively absorbs shocks, while the PETG provides a durable and visually appealing exterior. The two materials bond exceptionally well, and I also incorporated a mechanical interlock to ensure they remain securely joined over time.

The design also ensures a tight and stable fit for the upper metal cylinder, even if it becomes slightly deformed due to regular use.

For printing the TPU component, I recommend applying an intermediate layer or using ethanol to assist with removal. 

Due to its large contact surface, TPU can adhere very strongly to the build plate, which may risk damaging it during removal.