I just bought an Oregon Scientific ATC5K which can be mounted on either my handle bar, helmet or frame. Although the bar mount doesn't actually fit a 31.8 bar, the velcro strap and mount for the helmet/frame can be used instead. First impressions are good, although I need some practice before I start posting my results!
The GoPro range also have a chest strap set-up.
If you browse through the Video forums you should be able to get a good idea of what people use and how they use it.
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