This is the third solar oven that I've made over the last few years. But this is the first cardboard one I've made. This first pic is the oven without the lid and the second picture is with the lid on.
I still have to paint the inside with some flat black paint. I'll leave the top back 1/4 or so unpainted.
My first test will be without any insulation, just like you see in the pictures. Then I will try the hot box cooker approach. I will take a bigger box and a thick blanket or two and push this down inside so it will be surrounded by the blankets except the glazing section. Then I will add a reflector and see if it improves. Although a reflector mounted on the back of a slant face oven is almost pointless unless the sun is really high, or unless you have a flat top glazing.
And something to be aware of is that (as is true for all solar) size is king, or in this case, surface area of the glazing is king. Here is a picture of a small solar oven that has a 1 square foot piece of glazing letting the sun in.
With the reflector put into position and if it is facing the sun perfectly it will have 3.5 square feet of solar collection. You could make a solar oven with a slanted face that has 3.5 square feet of glazing and you would get the same efficiency without the reflectors.
Also, be aware that reflectors can get in the way. If you don't rotate that funnel type every 30 minutes to an hour, then you will get shadows from the reflectors.
Earlier testing with a thin plywood solar oven with no insulation showed temperatures from 190F up to 225F on a sunny summer day. I expect about the same with the cardboard one.
I'll keep you guys posted.