when using mylar its important you get all the creases out or it will cause heat spots and burn your plant.
so i fix mine to hardboard,i cut the mylar a little bigger than the board fold it round the edges and tape in place then i slide my board in place you can wipe it clean and keep using it.
p.s. cover the hardbosrd in plastic first so you can wash it clean.:thumbsup::thumbsup::thumbsup: