This is designed to fit Rocket828's "Printable LED Arc Reactor" (https://www.thingiverse.com/thing:12882), to be worn under a shirt.

As part of my Tony Stark costume I wanted an arc reactor under my tuxedo shirt, but didn't want bare wires against my chest. So I quickly drew up this simple clip holder, which can be used with a 4cm wide belt or strap.