I am gifting lithophanes to my mother (and mother in law), and did not like how they looked when lit using the nightlight brackets I found elsewhere. The main problem was the distance from the nightlight -- it seems like most brackets hold the lithophanes pretty far away to avoid the heat from regular nightlight bulbs.

Since the nightlights I'm gifting have LED bulbs and put off very little heat at all, I left only a short distance (about one knuckle's worth) between the bulb and the lithophane.

I print my lithophanes from regular photos, no cropping, with their longest dimension resized to 105mm (this puts the other side between 77 and 80mm for most shots). I print with a max thickness of 3mm, and when using Cura to quickly convert JPGs, I turn smoothing way down; just the slightest bit more than absolute zero.

This frame's gap is exactly 3mm to accommodate my 3mm-thick lithophanes. If you print them thinner, just use a piece of filament, a paperclip, toothpick, etc. to fill the remaining gap. Don't resize this bracket! Making it larger or smaller will prevent it from fitting standard nightlights.

I'm including photos here of the most basic, CVS-purchased nightlight I could find. This bracket fits snugly without any stress once it's on.