My model fixes 2 issues with the original diffuser for bambu lithophane frame - making lithophanes too dark and rattling in the frame.

My design has a relatively thick frame for strength and a thin “window” in the middle to let more light through while still diffusing the light. It's also 0.5mm wider to help with the rattling.

It's available in 6 thicknesses: 0.2mm, 0.4mm, 0.6mm, 0.8mm, 1.0mm and 1.4mm (as thick as the original diffuser). I recommend starting with the 0.4mm or 0.6mm diffuser and checking if you like the results it provides.