When heating acrylic and or plexiglass,
heat oven to around 250, then set the acrylic in oven.
Pay attention to it!
The sheet will heat up over the whole piece and it will shape quite easily.
You may choose to support the sheet on the corners, if you allow the sheet to sit on the rack, it may scar the finish from contact.
if you pay close attention, you will see the sheet begin to sag ever so slightly.
Wear gloves and have a wet rag close by to set the shape once you have the desired shape.