Making a heater box

What design considerations would go into a heater box? I need it to blow warm air on the windshield to prevent fogging/stop water from freezing on the windshield.

Is it acceptable for the heater box to take air from in the car and blow it through the heater core and onto the windshield? Or do I really need the dryer air from outside?

Thanks!

It's best to use outside air. It is fresher (less possibility of exhaust gas build- up). Initially, outside air is not much drier, but recirculating inside air does tend to increase humidity, unless it is drawn through an air conditioner evaporater.

Okay. Right now, I'm removing the (big) heater box that held an evaporator for AC. Is it typical to have to remove the heater core to remove the heater box? I'm considering cutting the plastic heater box apart.

yes, typically the heater core is installed either in the box or related ductwork.