they are very expensive to buy but essentially just use a widely available Philips infrared heat lamps 250watts each. these are incandescent bulbs and low EMF by their design.
so I fitted 5 to a backboard, wired them into a 3-way switch so I could have 2,3,4 or 5 on at any one time, housed that inside an untreated wood frame and covered that with untreated natural cotton canvas. the frame was hinged so you can fold it out of teh way to some extent when not in use
it got up to temperature in under 10mins even in winter and was as powerful as anyone would need - you basically just sit on a stool around 3feet from the lamps for 20-60mins at a time.
had it in the spare room for a while but the room was small so it wasn't easy to get a good photo.