yeah i guess i would have to agree,if you cant afford mylar or if your walls are already white..
but if youre like me and have grey walls with rose trim(dont ask,it was my wife's idea),what i would recommend is white styrofoam,like the ones that come with air conditioners,they are pretty big,or if its a last resort aluminum foil,but yeah flat white paint is pretty good,and styrofoam has the same reflectivity as flat white paint so either way you cant go wrong,but as for regular mirrors-folks say thay dont work because only the inside layer of the mirror actually reflects,and the glass from the mirror actually hinders the reflectivity so it kinda defeats the purpose