Clear waterproof coating to protect design

My friend is drawing this design on a plasticy-smooth part of my backpack with sharpie and it looks so good, but I dont want it to rub off if it gets wet or just from general use. When It's done, I want to coat it with something to protect it, but I dont know what. It obviously has to be clear, but it also can't smear the sharpie, and it has to be flexible. Any ideas? Would love to be able to use it on my helmet too, want to do something similar.