The original beta testers have said anything perfectly flat that covers the bed and is of the same whitish color the glowforge can score should work. If you use your own masking you can always keep remasking whatever you use. The reason they say draftboard is because they know it is large enough to cover the entire tray (at least the mostly cutable part) and scoring the masking gives the appropriate contrast. It seems the key is 1) the glowforge camera has to be able to see the marks it makes, so contrast and 2) the calibration software seems to assume the marks are at the same exact distance above the crumb tray, so perfectly flat.
I ran the calibration using 1/8" black acrylic, non-PG.
Before doing that though, I compared the settings and focus heights between draftboard and PG Medium Black acrylic - the settings are close enough for government work so I went for it. It came out excellent.
I didnt want to sacrifice a chunk of acrylic but… oh well, I can still find a prototyping use for it.