The good, old-fashioned pantograph engraver would be fine:

Paint the back of a sheet of Perspex black, then engrave the letters through the paint.

Simples.

No doubt there are sheet materials available specifically designed for this kind of thing...?

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Like this: http://www.mastergrave.co.uk/catalogue...ory_ID=123
[/edit]


Or, as Steve says, NC panel engraving must be a common enough task that there would be an easy set-up to do it.