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I'd say you're unlikely to get much smaller than 10cm, given the average portafilter is about 6cm diameter, you don't want to be forcing your customers to have a too much accuracy when swinging their portafilter!
Can definitely see the point of standard tupperware looking a bit naff on the counter top, but it's probably worth looking into things that aren't explicitly knockboxes for the stove-top.
My initial DIY thought led me to this (admittedly off-topic) creation: https://www.instructables.com/Pringle-Can-MIDI-Drums/
That Motta one you linked is 10cm, is that definitely too big?
I've got one of these: https://www.cafelat.co.uk/products/small-tubbi at just under 13cm and it's definitely on the small side!
It's been a while since I used a stove top, but do you need an actual knock box? Or can you just pick up any old cylindrical bit of tupperware that you can drop the grounds into?