How did you plan to engrave on it? By removing the cover or leaving it attached?
The thickness that you need to enter into the Unknown Materials slot will be the distance from the surface of the material to where the top of the tray would normally sit.
If the whole thing is thicker than about 1/2 inch, you’re going to need to remove the tray and set up a support to bring the whole thing into the correct range for engraving.
We’ve got a couple of tutorials that explain it in more detail.