http://castingaround.anthonynaples.com/2009/09/colorado-expedition-part-ii-the-taylor-river
I have resorted to cutting the post of a less expensive vise and inserted it into a purchased pedestal. This is not a bad idea (assuming the probable rock) and spares the hacksaw to the vise. Perhaps somewhere there is a creative sort that makes a larger C clamp for the typically thick picnic table?
SwittersB & Exploring 
Just to be fair I want to give credit where credit is due. It was my friend Larry’s idea. I’m not usually that clever.
John, that idea would probably work.
Please check out the whole blog post, where that pic comes from, for a report on my trip to the Taylor River if you’re interested.
http://castingaround.anthonynaples.com/2009/09/colorado-expedition-part-ii-the-taylor-river
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Another idea, a vise shaft that tapers down a wedge, like a screw-driver. This could be jammed between the boards on the picnic table.
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