I’ve done lithophanes in Acrylic, and it works pretty well especially with a narrow engraving v-bit or very small ball-mill/tapered ball-end mill (also frequently known as “rib-cutters”.
Eh, what the heck, here have a look at the Golden token ratio piece if you’ve got Fusion360:
http://a360.co/25nhYkj
I was playing with the relationships between various rectangles & circles all related by the golden ratio, and then did some drafting to create some interesting vertical relief between the surfaces.
Have fun!