Fishing wire comes in UV reactive flavors (purple is one of the colors if I recall) for visibility of the line in cloudy conditions.

Another thought is an acrylic rod (comes in various diameters 1/8, 3/16, 1/4 depending on how much weight you are holding up) which you can cut and would go from card to bottom of case. Cut to fit would be easy and you might even be able to mod the bottom with a LED to add color.

Nothing says you can't get creative when it comes to card support.