Convert AirTag to flat card
One of the complaints about the AirTag is that it only has one shape, which does not easily fit in most wallets. If you are handy with the soldering iron, you can follow the example of Andrew Ngai. He came up with the idea that the circuit board (PCB) and the battery can actually be split from each other, so that you could get a flatter shape. In the AirTag they are stacked on top of each other, giving the tracker a thickness of 8 mm. If you detach the two parts, they will each be slightly more than 3 mm thick. When placed next to each other you could make a flat card. Andrew used his 3D printer to make a holder so that the electronic parts and the battery can be placed side by side.
What's nice about the video Andrew made is that you can clearly hear his doubts about how he has to go to work, but you can also hear the enthusiasm in his voice when it turns out that everything is still working properly. Andrew took an AirTag out of its packaging and removed the plastic so that it is in pairing mode. That way he could check whether everything was still working at every step of the process.
What you need is a heat gun or paint burner, a soldering iron, a 3D printer and some electronics knowledge. You may be destroying your AirTag with this, so be prepared that it won't be as easy as shown in the video. Andrew took apart the AirTag, soldered wires to make sure it connected to the battery, and put everything in a 3D printed case. The result is a flat card that is just over 3 mm thick. The CR2032 battery determines the end result, because it is 3.2 mm thick. In the video, Andrew also explains in passing that with CR2032 the numbers stand for 20 mm diameter and 3.2 mm thickness.
Removing the back cover and battery from the AirTag is quite simple, but the parts are glued to the plastic case and that took some effort. With the heat gun at a temperature of 65 degrees Celsius it was possible to detach the PCB. The biggest challenge, however, was getting the PCB to reconnect to the battery. For that you have to solder some wires in the right place.
Alternative: a Tile Slim
Andrew went one step further: he uploaded the STL file for the 3D printed card holder on Thingiverse, so everyone can get started with it now. If you want a thin card for your wallet, you could also take a look at the Tile Slim. It requires no soldering and is 2.4 mm thin. If you are not a fan of Tile, then you will have to wait for a flat version of the AirTag.