I have struggled with this tool to be honest. Now that I have something presentable, I still am not happy with the lighting. I have a diffuser printed in generic white PLA and because the light panel does sit on the back of the panel due to wiring, I am still getting the LED grid showing thru. Any recommendations for printing a diffuser that actually diffuses?
Latest “Fixed Frame” and printing B&W lithophane.
I waited a few days for more knowledgeable people than me to chime in. Didn’t happen. So I’m offering a thought based on dealing with LED light strips:
Quite likely you need more room between the LED panel and the diffuser. That distance optimally should be the same as the distance between LED emitters.
Your other options would include some sort of prismatic diffuser (you’ve likely seen them in use in drop-ceiling fluorescent light fixtures) or a much thicker diffuser with the associated loss of transmitted light.
I just turned up this that also addresses the need to better diffuse for a lithopane:
There are others but people seem really happy with this one. So maybe try it before rebuilding your frame. 
If you are limited to the original frame thickness (thin) there isn’t a lot you can do with printed diffusers irrespective of material, but if thickness isn’t limited I wonder if you can make a 3 dimensional structure aligned to the LEDs which does a better job.
You are correct. The newer frame design has some improvements but still is very flawed in that the LED panel does not sit flat in the back of the case so the side with the wiring is tilted too close to the lithophane and shows LED artifacts. I printed a white diffuser doesn’t help at all. I printed a gray diffuser which cuts down on the artifacts but also cuts down on the light. I have been fiddling with this tool for more than a month and I have had so many problems with the generated models that it is unlikely I will ever use it in the future.
That sounds like a not very fun experience. 
sorry to hear it