Get another box fan, use one to blow into the attic, and one in the other window to suck it out. The trick is not to fight the wind outside.
As far as the plastic, buy some plastic sheeting (your local paint store sells rolls of very thin plastic sheeting that you use to mask windows) and use 3M Blue Masking Tape (this stuff won't leave a sticky residue, nor will it damage the finish) to tape the plastic to the back of the door, so it doesn't look funny from the bedroom.
Then break down and replace the door.
Since the attic is finished, I can't suggest more insulation, although you might look to see if the roof is properly ventilated. If you don't know what I'm talking about, write back.