Hi Ivan
How about this? This is the Navitron air heater panel. You need to keep the ducts between panel and room as short as possible (air has very low heat capacity). It only works well when the sun is shining, but I've found this to be fantastic in spring and autumn. In the winter, it works well on sunny days - here it is on a day when the temperature was well below freezing, and the room was warm as toast, despite some snow on the panel.
How is this plumbed in? If you draw air through with a fan suggested in the product blurb, how did you adapt from 50mm connections?
Mark.