i dont know where to buy mylar plastic personally so i would recommend doing a google search!as for it being a good idea!no its not a good idea its the best!mylar is the most reflective material i have come accross or that stuff thats black on 1 side n white on the other,i forget the name!the nitrogen thing is a good idea but your plants need to be able to absorb the liquid fast as it perspires through its leave and this can cause clogging which you do not want!also be sure that if you are foliar feeding(spraying the leaves)you cut down the nutrients you gave via root feeding so as not to over fertilise!good luck!