Compression is the key. I have a FF down vest; packed into a buff, then turn the buff sideways into another buff, perfect for side sleeping. Soft yet firm. Amazing pillow. Terrible height for back sleeping.
Key is having the right size stuff sack for your down item, be it a buff or whatever. Get some fine fleece from Joanns and experiment with different size stuff sacks (or whatever) to compress the down item to the right firmness/size. My FF down jacket would need a larger "container" to get the right height/firmness. Guess you could experiment with plastic bags, tying off as you go, instead of sewing until you get the right firmness.
Air pillows: In a single chamber air pillows the air just moves from side to side as your head adjusts. Instead, use two air pillows in your favorite covering (stuff sack, fleece stuff sack, etc). Your head will be cradled.
But air isn't soft. At least not at the pressure our head is going exert on it (or our body when in a pad). Air just moves around, equalizing pressure on the inside. So add some clothing (like a rain jacket) on top.
Air pads are great to support our bodies from the ground, but are not really soft. Foam pads are soft, as they compress when pressure is applied. Down is soft, as it compresses when pressure is applied. Air is hard. If contained in volume, it doesn't compress very much when pressure is applied.