Cover part of that top with wet towels. I would take that heat lamp off, unless you need it (and it sounds like you do). So, put a dimmer switch on it, then connect it to your rheostat. You will be amazed at what wet towels can do. Where are you measuring your temps from (where is your probe at). You can also try to remove the lamp and cover 3/4 with aluminum foil. That will keep your heat in. I'll get a pic of mine to show you.