The Flyte Park Microvario, I have it for a long time and used it as a backup vario. Mount it on the helmet and you are good to go. It uses CR2023 battery and they last for at least 1000 hours. :)
The vario is old but works fine, the only thing I could find was this video: https://www.youtube.com/watch?v=Z460ZI_a7QU