PRINT WITH FLEXIBLE FILAMENT!
I want to thank Gustav for providing this model of an airtag https://www.printables.com/model/255519-hi-res-apple-airtag-model-for-fusion-360-and-more.
This is my design for an airtag holder for a standard pet collar.
I don't actually own an airtag so I 3d printed the one from Gustav just to make sure an airtag could be inserted.
I recommend inserting the collar from the back to the outside so that most of the airtag holder will be hidden behind the collar. This will help insure the airtag doesn't slip out incase your pet likes to play hard.