But redditors, as they are wont to do, came upon a solution that harkens back to the original TV cartoon. As laid out by OP smithnigel, the original Eevee trainers were named "Rainer, Pyro, Sparky," and "they owned a Vaporeon, Flareon and Jolteon respectively."
If you name your Eevee one of those names, its evolution corresponds:
- Sparky for Jolteon
- Rainer for Vaporeon
- Pyro for Flareon
In the comments, another redditor posted a video showing the evolution on his phone as it happened, just to document the situation.