border – Draw a black border around a Plain TeX page
The border is drawn (as it were) “on top” of the page; since the border doesn't honour the \hoffset/\voffset positioning of the text, this is less of a problem than it might be.
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page trim marks and decorations
Maybe you are interested in the following packages as well.
- cropmrks: Add crop marks to a Plain TeX document
- placeins-plain: Insertions that keep their place
- cropmarks-pt: Philip Taylor's cropmarks macros
- cropmark: Crop marks on \shipout