Kinra,
Hey I deal with this issue all the time I too have an open rack my case is that the walls are so poorly insulated I can't heat it up really.
Anyway I have some temporary ideas that may help. Try to insulate the rack some blankets or towels especially the top and bottom if the floor is cool. if you have vent holes in the back you could drape a blanket over the back as well. This should help to hold some of the heat the flexwatt is producing in the rack rather than escaping to the room. It might be worth a degree or two on the cool end.