I used an inflatable rubberized fabric camping pillow for a while. Just bought a rubber inflating bulb from a medical supply house and connected it with some plastic tubing. Worked great. The bulbs are available as replacement parts because they eventually crack and fail, probably faster in a cockpit environment than in an office.

Two other reasons I stopped using it. First, the firmness varies with altitude, so you can find yourself adjusting it.

Second, I started to worry about how it would behave in a crash. Would it fail catastrophically just when I needed lumbar support the most?

I went to the memory foam approach. Not as easy to adjust. But when it's stuffed behind your back, it doesn't harden up in the cold.

Chip Bearden