That's clever! I like it. You may want to try a black pan though and maybe a three-sided cardboard reflector, covered in foil, to slide around your cookie sheet. It will reflect additional rays, hold in the heat better, raising your temps. I'd advise a cotton pad/sheet or even another piece of cardboard under the black pan to protect your dash. You may also want to crack open your sunroof to allow the cooking gases a means of escape out of the top.
Aluminum tape will be your friend to hold it together.
I built a solar oven many years ago as an experiment and was amazed it would heat to ~ 350°F in about 15min. To the amazement of some neighbors, I cooked a 9 x 13 pan of lasagna in it!
I ought to haul it out and try some cookies. Actually I could roast hot dogs & onions, then do the cookies for dessert.