Make enrollment automatic when you update your license when you turn 18, and store your key in your license with an NFC chip or something, so people don’t lose it. And maybe instead of a fine, do a tax credit or some other kind of bonus if you do vote. I think this is one of those situations where the carrot works better than the stick.
This all requires the government to be technically competent and move their technologies into the 21st century, which unfortunately means none of this will happen this century lol.