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Just started an identical undertaking, all I was able to find was this:https://www.theheraldrysociety.com/articles/some-arthurian-coats-of-arms/- although many of Mr Meier's figures are cast with differing heraldry.
I would just make it up and do what I fancied.No need to follow the laws of Heraldry.
Nice information, but if you want the fun element (as in Fistful of Lead threads)a friend I went to school with (Many many years ago) did his own list of King Arthur's knights:-(A few examples that I do remember)Sir Pent (Snake), Sir Vis (With a smile), Sir Cull (Round), Sir Vile (If that is OK with you), Sir Round (Attack from all directions), Sir Vive (Last to die), Sir Ender (First to give up),Sir Loin (Beefy guy), Sir Fet (Had enough of him), Sir Charge (Very taxing), Sir Face (Looks OK),Sir Geon (Cut out for war), Sir Vey (Looks over the battlefield), Sir Real (Don't believe what he looks like),Sir Row-Gett (Just a stand in), Sir Prize (Didn't expect him), Sir Plus (A bishop) & Sir Lee (Never polite).
The hardback version of the Pendragon rpg has it's own system of heraldry, illustrated on the inside cover pages if I recall this correctly. Sadly I got rid of mine a while ago but a look at the Pendragon/Chaosism website may be of help?
I would argue that King Arthur, being one of the Nine Worthies was history. Maybe obsolete history...
Probably the best book on Arthurian Heraldry is: Armorial des chevaliers de la Table Ronde : Etude sur l'héraldique imaginaire à la fin du Moyen Age by Michel Pastoureau