The subject is fascinating; but remember location can make a large difference even in legal status of certain arms.
It is of course considered poor form and bad manners to just 'assume' someone else's arms - in the British and Western European traditions. But Polish arms, for instance, in many cases were born by many people of the same surname at the same time. In America, other than as trademarks or copyrighted material, they are completely unprotected - I could fly a flag or paint a shield on the door of my truck that, in heraldese, would be tantamount to a claim that I was the King of England, or the Duke of Savoy, and so on. It would be perfectly legal. In some countries, especially those which still have an hereditary aristocracy, that would be illegal.
I made my own COA some years ago; a simple design. I didn't realize it at the time, but it is in my old school colors. Since I am an American, I can, in essence, grant my own arms to myself - there is an American College of Heraldry, but they have no enforcement power, they are an advisory body which will (for a small fee) register your arms - but they can't do anything if someone else wants to use the same arms.
The SCA College of Heralds is all-powerful, but only within the Society. They do a great job, though it is always too slow - if you are the one waiting. They have simple, logical rules; one of them is that they won't register "real-world" arms, even if you can prove they are yours. They will only register them with a 'difference' - some visible change in the arms. Of course if you don't tell them, and the arms meet the Society's rules, they aren't going to track you down later... (N.B.: I am not a herald; if any of this, especially as pertains to the SCA, is outdated or flat wrong, I welcome correction.)
I hope to have a full achievement, with mantling, supporters, motto, and crest ready when I do the full repaint on my vehicle. A full-color rendition on the door would be perfect.