I once thought of using flexible solar panels. I stopped short of ordering them because there was no reason.
With a 9V rated at 600mAh you should get over 10hours run time for your altimeter. Of course since a 9V would not fit my Estes Commanch I simply used a 20mAh NiCd cell and even that should give more than an hour.
Of course if your powering a video TX or other such electronics the umbilical is great.