For a 41mm diameter mini hygrometer  to be displayed inside a Stacker F-box while sealed inside. This slides into the tracks where the desiccant packs go. So using this hygrometer you can have a maximum of 2 desiccant packs inside the F-Box.

You'll notice one side is slightly longer than the other, the shorter side is inserted first. You can push this insert behind the pegs that stick out from the back to jam this in place so it doesn't shake around.