90 degree v bit, only .009 cutting depth.
You just need to scratch the surface, the deeper you go, the more detail that you will lose.
I have found that the recipe for successfully etching plexi is slower router RPM's and faster feed. This minimizes the heat buildup that can [will] cause plexi to weld itself back together, and build up on the bit tip. I had a slight build up on this one, which caused the details to appear less than stellar.
I use about 8,000 rpm and 200 ipm, vs wood I would use 16,000 and 100 or so - Don't forget to "flip" your image in your cutting program, unless you want the cut side to be the display side.