I made a simple app called VolumeSwipe. In selected applications (currently: Amazon video and music players, Netflix, and one more) it lets you swipe up and down on the left side (you can switch to right) of the screen to adjust the volume.
I know there is already a free app like this. But the nice thing about VolumeSwipe is that it's not active all the time, only in selected applications, so it doesn't bother you.
It's now released
here.