Thank-you Art-Tech for a really solid design! Here is my remix...

I added a second set of holes to allow me to join a series of frames together. I allso resized it to allow for 100x100x3 lithophanes since most of my boxes are that size. The slot is 3.5mm so it will hold up to that size border. With the open top, it is easy to switch out your lithophanes easily. Awesome for the grandmother with new pictures being updated regularily.This is a great design to make in different sizes so I have include the math below. Just adjust the sizing in the x or y axis so you don't effect the slot size to accomodate whatever size lithophane you want.

I printed it on its side without support. There was some stringing because of the bridge but I don't like cleaning support material. It's up to you if you would like that support or not.

To join the frames, I used some jewelry wire and crystal beads to catch the sun and give a sun catcher look. This is where you can custome it any way you want.

The top 3 Lithophanes are in esun cool white, the bottom one is in priline white. all done with .6mm(min) - 3mm(max) widths with a 3mm border. The cool white needs the sunlight to light it up well but the priline only needs the glow of the moon on the snow to light... you choose :)

measurements to note.... yes the math...

UPDATE TO NOTE::: I have discovered that if you just expand in slicer, your channels change size too... so don't count on the math so much... sorry... if someone else has a better solution please share... :)
take your lithophane size and adjust as follows

from top to bottom... +4mm
from side to side........+12mm

ex. 100x100... should measure 104x112 on your x and y
150x150... should measure 154x162 etc..

Time to start thinking about mothers day!