Fixxer that is a good idea but im broke lol i have black plastic that is really thick but im afraid it wont reflect
black absorbs light. You can use flat white posterboard but I think that's more expensive than paint and probably the same price as mylar
Multiple layers of white plastic trash bags if your ghetto. I would choose mylar over paint. Get some emergency fire jackets, those are made of mylar and can be found for free sometimes.