I've been very happy with http://www.flytec.com/Products/Variometers/Sonic.htm
It straps nicely to my shoulder strap near my ear. Life of the internal button cell is over 100 hours.

It has helped me stay up longer and has rapidly paid for itself by reducing the number of tows. Plus it has improved my scanning for traffic.
It works well in an ASK-21 and Blanik L-23.