My model is the thinnest rendition of the "AirTag Card'. It has no top or botom by design. You secure the expanded AirTag to the card by wrapping it in electrical tape. A good wrap should keep the internals protected from splashes of water. 

There is two versions of the AirTag Card. The one I use daily is the one with the straps for an elastic band. That way I can fasten it to my wallet and double the AirTag as a cash strap as depicted in the images I provided. 

The second version is simply the card with no straps for a conventional wallet. Whichever one you make, it is the absolute thinnest you can make an AirTag.