Here is my review of the Breville BCB100 Barista Style Knock Box https://amzn.to/2ZKVZYa.
The BCB100 features a two-piece design, a cast aluminum outer layer/base and an easily removable plastic container with knock beam.
The knock beam is non-removable but very easy to clean and is coated with rubber to reduce the noise.
The aluminum base is heavy and provides a solid footing for when you need to dump your used coffee grounds.
In my opinion, a great buy under $30. You can get it used too for even less.
The size of the knockbox is great for people who make 3-4 shots daily. It can hold a few days worth of grinds before you need to dump it out. For people who make 1-2 shots per day, the smaller $20 Breville BES001XL Knock Box Mini, maybe a better buy. https://amzn.to/2ULoxuS