This is the smallest altimeter available.
It's been used in rockets for recovery for well over a decade.

It's not the easiest as you have to solder to the board. But for the ultimate in weight savings it's worth it.
If you can read a schematic your all set.

This altimeter has two outputs, one for Apogee and one for Main (low altitude).
It uses a barometric sensor to detect apogee and to determine the Main deployment point.

It also has peak reporting to give you the height of the last flight.

All powered by a single LiPo battery.