Below is a link to the user manual for a Digital Tension gauge for
paragliding and hanggliding winches but it might be adaptable for auto
tow of sailplanes.

http://www.lemmen-engineering.nl/ima...Manual_DTM.pdf

Excerpt:
The Digital Tension Meter is specially designed for paragliding and
hangliding winches., however it can
also be used in other applications where force need to be measured.
The Digital tension meter is available
in two versions: single and double cable drum winches (DTM-S and DTM-
D).
The digital tension meter consists of three main parts: the analogue
panel display, the DTM box
(electronics) and one or two load cells. The DTM can be calibrated to
the application by means of three
buttons. After calibration the DTM stores the calibration values in
non-volatile memory.
The DTM-D automatically compares - after calibration- the measured
forces of the sensor inputs and
displays the highest force. The display value is updated every 25ms.
snip