Idea of Safe Icon
There is a safe shape with a square and a circle in the center of it. The outer square has another smaller square towards the outside. The circle also has a smaller circle within it. There are dashes on the outside of the larger circle. The outer square has two small rectangles on each side.
This page shows the different variations of the Safe from an Icons8 icon pack. Feel free to resize and recolor the icon, and then download the icon in PNG format for free.