a 400w light on 18 hrs a day will cost you about £2 a week.
how high are you planning on building your box, my guess is that it will have to be at least 6ft tall. to allow for growth of plants and the distance from the top of your plant to the light.
i cut a piece of plyboard to exactly fit into my window frame, i closed the curtains and squeeze the board in and no light escapes. it just looks like your cutains are drawn all the time.