I got this really cheap case from Amazon:
and like it a lot. It is stiff plastic for great protection and the phones fit inside snug and secure wearing the factory case. It is minimal size, closely following the contours of the headphone. There is a deep recess between the headband and the drivers that makes a useful handhold. It looks OK. There are others that look identical and are even cheaper! Good enough for me.

AirPod Max Case Compatible Smart Case, Headphones Storage Bag, Headset Travel Carrying Cover, Protective Hard Shell (Grey
The Carrying Case Perfectly fits Airpods Max Headphones and Compatible Smart Case. Perfect fit to the Headphones and other small accessories normally used with these Headphones fits very well and at the same time does not waste your valuable space.
www.amazon.com
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