Thanks for the replies!! I had a thought of something I might try. There is a relatively new fly fishing technique that uses a "sighter". That is a length of monofilament line that is wrapped around a dowel and put in hot water for 30 seconds or so, and allowed to cool. It sets in a spiral like a spring. I bet a piece of that in 10# florescent yellow would show up very well. I'm going to try the small cork, and just a piece of 10 or 12 # floro mono set at 4 or 5 feet.
Also, Greg Fenn of Camo River Guide Service has some good videos on you tube about using Academy lime green catfish line and marking it with a Sharpie. Also several videos on Panoptix Livescope, and on putting out brush piles.