Lock Computer Automatically When User Leaves Desk

I have been tasked with finding a solution that will lock a user's computer when he leaves his desk. We already have group policy configured to lock workstations after 10 minutes, and we instruct everyone to lock theirs with Win+L when walking away, but this user never does.

I have come across a couple of different options. The first would be to get a device similar to one of these, but it would of course require the user to carry the device with them at all times:

http://www.vupointsolutions.com/ez_secure_comparison_chart.php

The second is to get one of these temperature sensors, and use this program "LockIfNotHot," which optically measures the temperature of the user's chair, and locks the computer when the user walks away:

http://www.phidgets.com/products.php?product_id=1045

http://blog.didierstevens.com/2011/04/06/lockifnothot

I'm curious if anyone here has had to come up with a solution for locking a workstation when the user physically leaves, and what suggestions everyone might have.