Ive been printing CMYK lithophanes and noticed the website recommends 144x108 mm. Which is fine. I have some DSLR photos which are a different ratio so I kept the long side (144mm) and the short side is 96mm. I made this huge lightbox but I wanted something simpler. Hang this with a suction cup hook on a window and you can see the colors in the morning or afternoon depending where your window faces.

There's 144x108 both portrait and landscape, 144x96 portrait and landscape. No support needed. I would use PETG since this may get warm by a window?

Slide it in from the top, you can replace the lithophane as the mood suits you. Hang it with a suction cup hook.