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what type of heater? air, water, mass?
If you are doing air then just have a flat black surface a few inches from the window glass with an opening at the top and bottom with a flap to let air in at the bottom and one to let warm air out at the top, a fan on a thermostat would help, it all depends on how cheap you want to be.
the main advantage of something like this is that the back side of it can be insulated well so it's like having an insulated window shade all the time to limit heat loss.
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