I've been using a repurposed plastic box next to my espresso machine as my go-to knock box, but recently wrote a guide on knock boxes. I'm curious if you can recommend any particular knock box? They all seem universally the same and universally expensive.
Osaka, Shock-Absorbent Espresso Knock Box 4.7 Inch Barista Style Knockbox.
about $20 anywhere online
Knockboxes are practical and make it a pleasure to make espresso. I bought mine based on good reviews and after using it a few months consider it a great purchase.
It's made out of a good grade of tough plastic and can be found around $20 online or even less. Rubber coated knock bar and a well thought out cutout on the front for ease of access. Available in red, black and a few other colours. The stainless model is closer to $50 but is elegant if you prefer stainless and would stand up to genuine commercial use.
I use a Cafelat Tubbi Knock Box in small because it's cheap and a nice size. If I was going to replace it I'd get a Rattleware Maple Knock Box because it's solid and beautifully constructed, plus it's quite handsome.