I just finished the first prototype! The display now shows the current and max altitude.
If I gently blow on the BMP 180, the altitude changes so I'm thinking that the wind from the aircraft might affect the readings. I may have to put the device in an enclosure.
Also, I'd like to verify the readings but I'm not sure if I should bring my phone up that high.
Now I'm off to charge some batteries and do some testing!