Garbage bags.. doubled up.. Thats the cheap method.. You could try aluminum foil, but it is less durable and will break on you. If youre growing, then youll want to cover windows too and it HAS to look inconspicuous.. You cant just board up the windows, outsiders will get very curious... And might call the police, who will send a chopper over your house looking for heat sources. Best thing to do is put normal blinds up, so no one outside knows the better.. Then, on the wall around the window put little nbs of a 2 by 4. Then cover that in cardboard and nail it into place. Light-seal the holes on the side. Boom, youre a phantom. No one is the wiser. This will also prevent smell from getting outside for an unsuspecting peeping tom-crimestopper to hit the nineunouno. Good luck to you sir. If I see you on the 6 o clock news, I will castrate my dog.