The only way to change the volume with one hand is to use Siri.
But triggering Siri may cause you to miss something of the audio that is playing (especially when it's an audio book).
I've configured my left AirPod to play/pause, and the right one to trigger Siri. So then:
- Double-tap left AirPod to pause.
- Double-tap right AirPod to trigger Siri.
- Say "increase volume", "decrease volume", "volume to max", or something like that to change the volume.
- Double-tap left AirPod to play.
By default, both AirPods trigger Siri. You'll need to change the double-tap action for the left AirPod. See https://support.apple.com/en-us/HT207009.
Note: In the initial AirPod firmware, it was impossible to assign different actions to each AirPod.