In celebration of New Horizons’ fly-by today of Pluto, some hadeocentric symbols and flags:
Pluto’s astronomical symbol: a monogram for PL, standing for PLuto and for astronomer Percival Lowell who first found compelling evidence of its existence. The Lowell Observatory proposed the symbol and name on May 1, 1930. As text, the symbol raises difficulties for flag designers, because its reverse is illegible.The astrological symbol for Pluto: the symbol for Mercury, with arc and circle exchanged. It can be interpreted as “mind (crescent) transcending matter (cross) to reach for divine spirit (circle)“. Vertically symmetric, it’s a flag-friendly (if not science-friendly) symbol.A design by ONI-Defense (on DeviantArt): “The flag for Pluto is simple, it’s coloration representing both Pluto, ruler of the underworld, and the dwarf planet, Pluto being an ice world. The navy blue circles represent Pluto and its moons, Charon, Nix, Hydra, Kerberos, and Styx.”A personal flag by “Pyrrhos the Red” (on EstotericOnline): “A flag of personal value to and design from me. It has the astrological symbol for Pluto because 1.) Was one of three planets in the constellation of Libra when I was born (in October). and 2.) It represents Body/Physicality/Matter (Cross), Mana/Lifeforce/Energy (Crescent), and Ghost/Spirit/Soul (Circle). The colors also represent such (from black down to red), being a rough representation of the process of refinement in philosophical alchemy (putrefaction, calcification, etc.) and simultaneously representing my love for my German heritage.”A flag for Pluto’s largest moon Charon, by Maddish (on The Voice of Vexillology, Flags & Heraldry): “The flag of Charon uses four colours: purple, navy blue, black and white. This flag has a strong counter charge aspect. The taller figure is the God Charon, the ferry man to underworld. The sitting black figure to the right represents a person crossing over to Hades. The boat is also shaped like a moon to denote Charon’s bi-terminal status as a ‘moon.’ The staff of Charon is black and white denoting the elemental dichotomy between life and death. Near the upper hoist is the symbol of Pluto.”“Flag of Charon proudly flying on a ‘Pluto-Rise'”A flag for Pluto, and Plutophiles. Design by Scott Mainwaring, 14 July 2015.
Portland Flag Association 
The design Pluto and Plutophiles by SDM would make a great flag.