You already have one of those. If you are driving a 4-speed automatic, you can see that you are in 3rd and sense that it might be about to upshift and a turn is

You already have one of those. If you are driving a 4-speed automatic, you can see that you are in 3rd and sense that it might be about to upshift and a turn is coming up, you can whip it from D to 3 to hold it in 3rd for the duration.