I have traveled with Libre in my arm and Libre in a box.
The security at the airport don't bat an eye at either. They are not detectable and are no risk so I do not offer information regarding them
I DO tell them about my pump but only because it cannot be x rated
In short, take spare Libre however you want. Doctor's letters are not necessary for sensors
I don't have a libre but do take a bagful of meds with me including Schedule 3 controlled drugs. I pack them in a large transparent bag and take it out so they can see it. Most times the staff ignore it, although after avoiding the xray machine they carefully scan me with the hand scanner and test for other drugs! It takes about 5-10 mins extra for me to get through the checks so do allow more time.
Covid aside i travel to the canaries twice a year and all my type 1 gubbins including spare boxed libres are in my hand luggage with a copy of my prescription.
I've never had an issue.