How important is the Up-Down component?
I released an audio toolkit that was degraded to simply pan L / R on platforms where HRTF is not available. For many games, where most of the action tends to take place on approximately the same plane, this is more than enough to provide the correct positional signals to the player.
It's easy to implement: use the angle between the viewer and the source as an input to the constant power pan algorithm.
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