How can I create a custom border using the same material as the world boundary?

Pretty easy:

/worldborder

You can set the diameter by /worldborder set <distance>. For example /worldborder set 5 results in this (a square of 5x5 blocks):

enter image description here

Moving the border is also possible by using /worldborder center <x> <z>, which will put the border around these coordinates.

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