I liked the model Engagement ring box cryptex from tecnao but did not have any foam at hand. So I designed this insert to be printed from 95A Shore TPU. It does not go all the length of the box for ease of installing and removing. It holds firmly in place but because of the flex in the insert and the boxes sides it can be snapped it and removed if needed.

Printing:

Without supports, flat face down with sparse infill 10 - 15% and only a few perimeters to ensure the flex. You can even add a little text on the face in the slicer.