https://wccftech.com/drill-hole-in-airtag-attach-keychain/

The AirTags are either available for $29 for a single unit, or $99 for a 4-pack, but bear in mind that with either purchase, you will not get any accessories such as a keychain to prevent your trackers from getting lost. Fortunately, there is a workaround to that and it involves drilling through the AirTag, if you are bold enough.

A MacRumors forum member called smythey has shown proof that it is possible to drill a hole in the AirTags and allow for a string or keychain ring to slip into the hole. Using this method, you will not have to spend extra money purchasing keychain accessories to keep the AirTags from getting lost. The only drawback is that this will not be the most elegant solution and will definitely void your warranty in the process.