Plastics are difficult. Though there are some materials like gray depron or PU which work well, most of the time it is very color dependent. Acrylic works on opaque black and white, colors are engravable as long as the color is not blue. Depending on the chemical ingredients of the pigments sometimes it is creating bubbles.

We have some special engraving material made from ABS. This works quite good as well.
In general Mr Beam II is great for organic stuff.