This is the solar cooker I designed for my Calculus class. It's a paraboloid that results from rotating the equation y=.8x^2 around its axis, from the bounds [0, 0.25]. I included one full 18 degree slice, in case your printer has an incredible print area with one dimension over 25 centimeters. I am not so fortunate, but I was able to print one slice at a time by dividing it into two pieces. DO NOT START THIS PROJECT IF YOU CANNOT FINISH IT: If you want to print the whole paraboloid, you need to print 20 of these slices. You can get by with fewer, depending on what material you use to make it reflect light, I was only able to print 12 in time for my project, and as you can see aluminum foil does not stay smooth, especially when it is not supported. I recommend finding some kind of reflective tape, and making sure it sticks to whatever plastic you use first. Definitely print these on the lowest density you can without having the upper surface change shape, because that surface is the only thing that matters.
Why a paraboloid? Reflective paraboloids have the unique property of collecting all of the light that shines into it parallel to its axis, and focusing it to exactly one point. At this point, which should be at 31.25 cm directly above the center of this particular paraboloid, all of the energy becomes focused, and it becomes possible to cook things like marshmallows.
Please let me know if your cooker works!