100 degrees C for me.  Lay them on a baking tray with something like baking paper over the tray.  DO NOT put the plastics in front of a fan in the oven is fan forced, it will heat the plastics unevenly and whilst the front may look OK, the back near the fan will bubble very quickly out of nowhere and the plastic will be ruined.

As soon as the plastic starts to flatten out in the oven, remove them and lay flat on the kitchen bench with some weight on top of them.  They will cool down and harden in a matter of seconds so no need to leave them too long before you inspect your handy work.