this might sound like a daft idea..but mylar is free!!!! its on the inside of all modern food wrappers! for instance if you buy 3 large bags of doritos or similar brand you will have a fairly good size sheet of it! Ofcourse in a large grow its pointless to think this will help, but in a small scrog cab or c-13 grow it makes sense!
And you get loads of crisps!!!!!!
cheers!