This self-adhesive sleeve looks like a universal option, as it can be attached practically to any flat surface.
It has a silicone sleeve on one side and self-adhesive on the other.
The self-adhesive layer seems to be a good choice. It is not extremely sticky as some other products, it does not seem to leave marks, and I actually managed to remove the case from the Storm Box on our Tern GSD bike, move it to a more suitable spot and re-apply it.
It fits nicely inside the sleeve and as far as I can tell, there is no chance of it slipping out. The silicone keeps it from moving. In fact, it takes some effort to insert and remove the AirTag, which is great.
There are many accessories available for the GSD. Storm Box is a great candidate for an AirTag in this sleeve. Mine is still holding tight. It has a few internal pockets and you can easily attach a dozen of AirTags to it and still have some space left for more;-)
Bottom of the front rack might also be a good place for an AirTag.
A word of caution
Wherever you mount it, please keep your own safety in mind. Don’t mount it near the motor or anywhere near the chain/belt. It might cause you some bumpy ride should the self-adhesive fail.
Where can I buy one?
I bought mine on eBay and it was very affordable. Here is a link if you are considering getting one or two.