A compartment to keep your custom made ear plugs and replacement filters (ER9/ER15/ER25). The aim was to make a case as tiny as possible, yet robust, fit to a keyring and store an additional pair of exchangable filters. (It's quite handy to have filters with different dampening characteristics ready to adopt the environment.) Watch this youtube video about this design.

To adjust the size of the case use FreeCAD and edit the file "OhrStöpselRöhre.FCStd". Open the spread sheet "Masse" and adjust the values for TubeRad and TubeLen. You can also modify "Spiel" in order to adjust the clearance to the precision of your printers capabilities.