@Bigjohn: Maybe not enough detail…
I used one white and one black sheet of PG medium (~3 mm) acrylic. After adding identical sized “8551” letters to two different SVG files (one for each sheet) and converting the text “Object” to a path in Inkscape. I needed to “fatten up” the black letters in all directions by exactly 0.2 mm. This is how I did it using Inkscape (thank you @Jules)…
This seems complex, but in fact is quite straightforward. You turn your letter outline “strokes” into “fat” strokes, convert those “fat” stokes into two “thin” strokes and then delete the “inner” stroke. Simple! 
I reversed the letters before cutting using “Object->Flip Horizontal”. After you cut the white base plate and the black letters you will want to insert the letters into the backside of the plate. That’s because the laser’s kerf is not straight up/down. The laser’s cut is more like a cone than a cylinder.
If you do it right the fit is perfect—in my experience. YMMV. 
