You could always have it memorized.
You could also go on your mobile, to the notes app probably already installed on your device and have it written and stored there. Then safeguard it all with a password or whatever security lock your device uses.
This your method looks very risky and if we store our private keys in this form then we might lost it if our gadgets like phones or computer get hacked. We know that hacking of wallets is one of the ways many investors had lost there funds because of carelessness or lack of adequate info on how to protect there wallets from hackers.
I have seen so many ways we can protect our private keys from getting destroyed either by water or fire in that eventually occurs. There are tools and gadgets that are water resistant we can store or write our keys on that could make it safer from natural disasters or hackers.