An oven, a common domestic one is fine. (do check with the other half first) Fortunately there is NO smell associated with the process, you can even cook tea while the oven is still
Pre-heat it to 200deg C, or 390 deg F.
Arrange a metal dish or similar to support the styrene sheet and frame inside the oven.
Make sure that the dish is deep enough to allow for the sag that will occur in the plastic when it is heated. (maybe up to 30mm)