We use red electrical snap ties. Put it together, make a loop, cut it off to the right length. Insert down barrel and its real easy to see and super cheap. Example a length of 7-8" works fine on a 6" revolver!
Our club just uses off-cuts of whipper snipper cord (weed-wackers / line trimmers) when guns are left on the bench so shooters can move forward and score and patch targets. Cylinders left open naturally.