Honestly it's actually pretty easy you don't even need a popup on the screen:
When it's not your turn your toolbar flips to the reaction toolbar and if your character procs a reaction you just hit a button in the 2 second window. If you're too slow that's on you. You should know roughly what you want to do before hand, and 2 seconds is the perfect amount of time for shit like "I DIDNT KNOW I JUST REACTED" aka. Reactions.
This is something that looks great to me.
