I wanted to add my experience here. I had a moderately warped Space Shuttle, HOT NEW PINBALL topper plastic. I didn't want to destroy my sign, but, worst case, I knew I could buy another plastic set for around $150.

I cleaned the sign with Novus #1 first. Then I heated the oven to 170F (I'd prefer lower, but this was the lowest setting), and set a timer for 10 minutes. I put the sign, face up, in a no-stick, very smooth and flat roasting pan that was larger than the sign. I'd prefer a glass pan, but didn't have one big enough.

I watched the entire time. After a few minutes I could see the sign gradually flattening on its own. I left it in for the entire 10 minutes, then shut the oven off and opened the door about 6 inches to let it cool gradually. After the oven cooled down, I pulled the pan out and let it cool to room temperature.

The sign came off the pan easily and was perfectly flat. After careful inspection, I could not see any damage to the sign.

I still think this is a risky procedure. I wouldn't try it on something that could not be replaced.