We make chamber flags out of orange string trimmer line and plastic milk jug caps.
Drill a small hole in the lip of the cap, attach a three foot length of the plastic line, and you have a chamber indicator that will fit anything, and is highly visible.
I make these up while watching old movies in the winter. I drill the hole tight for the line, and after inserting the line through the hole I put a dab of silicone on each side of the cap. This give a secure bond, and is the simplest way I have found to make the connection.