NOTE: my 3d printer's not set up at the moment, so I haven't had a chance to print this. Please confirm if it works for you, and remix or suggest/submit improvements as needed.
This is a snap-on adapter to connect the Ambrosia Systems (ambrosiasys.com) Blucon Nightrider bluetooth transmitter to an Abbott Freestyle Libre (freestylelibre.com) flash sensor.
It's designed to be a lightweight, tight fit, which should keep the Blucon attached throughout the day (and night!) until you ACTIVELY TRY to remove it.
It's similar to other press-on / snap-fit adapters sold online, with the main difference being a small overhang and lip on the bottom, which snaps around your SENSOR as well as the transmitter.
Using a flexible/rubbery material like TPU will help with both the snap-on design, AND the comfort level of the lip between your arm and the sensor. Also use the finest resolution you can stand to wait on (at least once you've tried it and know it fits etc.) so the print isn't as rough on your skin.
Not sure if TPU can be sanded, but anything you can do to smooth it, like maybe smoothing with acetone, will help.