I modified this existing lithophane helper a little: https://www.thingiverse.com/thing:2718124

I added a little more stability of its own to the base, and sloped the contact bars to reduce overhang and bridging. The sloping is gentle enough at full size to not need supports, and even if you scale it down to 50% height, the angle is still not going to exceed doable overhangs.

Edit 2/23/20: v2 - Added a second version with a cutout center bottom, for those lithophanes with a wide base.

Edit 3/22/20: v3 - made version with chisel-point tips to minimize contact with the lithophane

The rest of the original info in the item above stands, and here's a video on using these: https://www.youtube.com/watch?v=WNzIMxbtNdY&t=13s