A variation on the heat gun method is to use 2 sheets of glass, place plastics on one sheet, heat them until they are soft, then place the other sheet on top, does an excellent job and is quite safe as long as you don't overheat the plastics. If you have enough direct sun, you can do this gradually outside, simply place the plastics between the sheets of glass, and let the sun do the rest!!
Couple of old sheets of scratched playfield glass works very well for the purpose!