It's rare but even an experienced Driver can blow seeing the checker and suddenly they're still going full bore on a cool down lap. The clue generally noticed after about half the lap is the Corner Workers are waving their hands "Hello!" to all the cars.
Idea........
Remembering the SCCA is the Sports Car Club of America
Workers still wave such that the Driver should wave back as a way of thanking the Workers. How about this > starting with about the third Worker station after Start/Finish, on the Cool Down Lap, all subsequent stations also wave the Stars & Strips when they see the Leader during the cool down lap. Not only would the Driver who failed to notice the Checker have an additional clue that it is the Cool Down Lap.....but this might also have it such that some other hasn't-got-a-clue Driver then doesn't blow the Checker and take an additional Cool Down Lap.
I thought to bring this up here because so many of you have insights I wouldn't think of on my own......and that way if I do write this idea up as a proposal, it might be a sounder one.