I am replacing a few panels of 'good neighbor' fence for a pal.
So I bought four of these:
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=43569

With cutting and grinding (and gluing) I transformed them into a
bipod that supports the end of the fence top rail in alignment with
the witness line.  By twisting each tube, I can 'dial in' the
placement of the end of the rail quickly and accurately.