This is a replacement case for an RFID fob. The original plastic fobs have a bad habit of not lasting that long. (See photos)  This works with internal elements that are 20mm or less in circumference and about a 1mm in thickness. I left the internal gap a bit bigger than the RFID element to allow for various chip mounting differences, etc. Note of your unit is has a broken loop vs cracking all the way open like the one shown, you may have to open your case to get at the element. 

This is a snap fit case. I recommend adding a dab of glue or tape to avoid it rattling around inside. Clearly you can also glue the 2 halves together on top of the snap fit as well. It's recommended that you have a secondary metal ring (like pictured) to put on this to give it more play on a keychain if that is how you are using it (the torque on the loop is  the reason most of these break to start). 

The unit shown was printed in red Kretrum PLA+ with the logo added right in the slicer. The files included are blank and can be either customized for your needs, or left blank for extra security.