You can use pieces of styrofoam to insulate a solar oven. Find designs at the Solar Cooking Wiki on Fandom. (It's a great website, check it out anyway).
They're made with aluminum foil, cardboard boxes, a glass pane. Use styrofoam between the boxes to retain the heat.
https://solarcooking.fandom.com/wiki/Principles_of_Solar_Box_Cooker_Design
