Need Help simulation flow design, open ends, no forced flow (oven type design) 

I'm creating a heated oven type design, 2 open ends to ambient air 

I want the glass heaters at 1800W and there's fiberglass insulation within the 1/8'' aluminum plates

I'm not sure whether to do external or internal there's no forced air at all

i want to find the temperature of the air inside (i need it to stay at 450F) i can mess with the Wattage later

also I'm having trouble selecting the open areas 

Please help, and thank you