On Tue, 27 Apr 2004 16:32:57 -0400, Glenn Ashmore
wrote:

Anyone ever seen such a device or have a better idea?

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This would be sort of expensive and hi-tech, but how about a fishing
bobber tied to a concrete block with monofilament line? Adjust the
length of the line so the bobber is just starting to be pulled under
to ensure accurate positioning.