One of the simplest designs possible, but ... it did not exist on Thingiverse, so here it is.

An OpenSCAD design to create spacer rings for vaping box mods. It generates 5 different sizes (default from 0.1 to 0.5 mm). You just need to measure the outside and inside diameters, change the C1 and C2 parameters, render and print at 0.1 layer height (preferrably with 4-5 outer perimeters).