That is one problem: if you only want one knob you have to buy a lot of materials that usually have fairly short shelf lives.
He makes it look easy but I understand there can be problems with air bubbles so often vacuum is used. Then for knobs with grub screws you need brass inserts although some just tap the plastic. This is poor as its relatively soft and doesn't take a screw thread well.
Here is a set of knobs made by someone who had the know how and experience (I suspect you make some bad ones at first

) Charged me 30 Euros (roughly £s) which to me was excellent.
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Wims knobs 1 web.jpg [ 52.62 KiB | Viewed 547 times ]
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Gary