Jay

OK, looked at the pix of the appt. Is thre an electric outlet outside? If so, you can place an electric heat tape, as Rips suggests, to moderate the temp. I use one in my unheated garage. I have the type that has an auto stat. I made a grid out of copper tubeing, filled the grid with auto anti freeze and placed the tape on the grid. You could do this, put the grid in the bottom of the box, wood would probably be better to use. put a couple of inches of peat moss on top of the grid and then bury the trees in some more peat. That would help. You could still put the box up against the window if it is 'politically correct' in your house. The auto stat of the heat tape will not let it go on till needed and will shut it off if it is getting warm in the area of the stat. By putting a few inches of peat under the trees it will not allow that much transference of heat to the trees. Make sure to allow for air into the box.. do not seal it! You want it cold.... just not THAT cold.

__________________
A Bonsai student living with his trees at N 44.37 W 77.49...
Think before you act... then think again... no good comes from rushing