My wife's dog had to be put down last month due to an unexplained illness. I made an outdoor memorial lithophane using a solar led spotlight. We plan on putting it where we will spreading the ashes.
It's best if you print the holder in a dark non-translucent material that can withstand being outside. I would also recommend that the lithophane be printed in the same fashion. PETG should hold up pretty well.
I used http://3dp.rocks/lithophane/ to make my lithophane. The model settings that I used are as follows...
Max Size 150mm
Thinnest Layer .08mm
Make sure that you download the "Outer Curve" version.