I secure mine on a clean usb flash drive and I also have a copy of it on a piece of paper but I always misplace it so I find it safer for me to use flash drive rather than a piece of paper.
But of course there are risk on holding it on such thing some would agree that the safest way is to store it on a paper because it could never be hacked online or be infected by a virus.
That is why I only use my flash drive when needed and I don't connect it on other device only on my computer.