Find a cheap but complete non working recirculating heater and make your own core. All you need is flexible 3/8 inch copper tubing...between 15 and 20 feet is what I used plus two 3/8" to 1/2" converters to slip inside the heater hoses and a bit of sheet metal to make a fan mount (really simple). Simply bend as needed in a tight circle getting larger as you go. I think (it has been over 25 years since I did this) I began wrapping around a coffee can and spacing each winding about 3/8 inch apart. Just be certain you leave space for air...but not too much space. You will be surprised how well it works, and because it is thick copper and seamless (aside from the adapters) it won't leak in your lifetime. You may not even need to use the fan...unless it was an abnormally cold day just the heat radiating from it kept me plenty warm. Good luck!
