Heat gun is good and you can see how it works in the video. If you have multiple plastics then I prefer the oven method since you can do a bunch at once. That will make the entire plastic dead flat since you are heating up the entire plastic and then flattening with weight on top and keeping the weight on it as it cools.

You set the temperature to a very low setting. The plastics will soften and lay flat before the setpoint temperature is even reached, so that is why it's important to watch them the entire time and the pull once they go soft. I will have to check and see what temperature I used (it's low in the 185-225 range). Since you never reach the final temperature you want to pick something low that way it has a nice slow warm up. I use parchment paper to keep any debris from the cookie sheets from getting on my plastics. When they are done I pull them out of the oven, place another cookie sheet on top, and then add some weight (typically use four 30 packs of pop). The combo of heating the entire plastic plus using weight really does a great job. Plus you can knock out a bunch at one time.