Over the years I've shared other sets of directions for making solar ovens. Here's a recap of those resources.
NASA provides two sets of detailed, written directions for building solar ovens. This set of directions (link opens a PDF) was created for students in 7th through 9th grade. This set of directions (link opens a PDF) for building a solar oven was written for 6th through 8th grade students and culminates with students attempting to make s'mores with their ovens.
Cooking With 'Sol (link opens a PDF) was published by the US Department of Energy. It was written for students in 5th through 8th grade to follow directions to create a solar oven.
DIY Sun Science is a free iPad app from The Lawrence Hall of Science. The app features directions for hands-on lessons about the sun. The lessons are a mix of activities that students can do on their own and activities that they should do with adult supervision. All of the activities use common household goods. Some of the activities that you will find in DIY Sun Science are measuring the sun, making UV detectors, detecting solar storms, and cooking with a solar oven.
If you or your students want some visuals of how a solar oven works, SciShow Kids offers this video for you.