I know there are a few HP fans checking this out and I also warned Alice that my contribution may end up being a bunch of arts&craft ideas so this seems like the perfect way to start.
One of my little sisters friends spent a lot of time in the hospital as a child and became a knitting prodigy. This is her newest creation. Its too incredible not to share:
So from straight on this just looks like your run-of-the-mill striped (black and glow-in-the dark green!) scarf but as you look at it from different angles the dark mark is revealed! I have no idea how she did this and I sort of want it to remain a mystery. Maybe she's a deatheater..