Richards Design Suggestions

Hi, Hef!
An interesting idea. I assume you are planning on using solar energy to supplement your heat.
I note from your tests so far that the back of the board gets warm - Excellent! This gives you another surface from which to get heat. And if this were the case, then why not maximize it with a metal 'absorber'.  Also, since heated air rises, why not make use of that and end your air path at the top. Given these suggestions, how about something like this;
Solar Panel Design Suggestion
Where the blobby stuff is insulation and the zig-zag is a sheet of corrugated metal, painted flat black.
The idea is that air enters at the top right side, behind the metal and is drawn down (pre-heating as it goes) and under the bottom of the metal by the convection on the front side of the panel. The flow can be assisted by a blower if desired. The heated air would exit at the top left, in front of the metal The back of the box would be painted shiny silver (or foiled) to keep more heat in the metal for the in-coming air to pick up.
The glass on the front could have a 'Low-E' coating to prevent some of the radiant energy from escaping out the front of the panel. This option adds cost, of course.
A flat piece of metal would work also, but I opted for corrugated to increase surface area. Also, it might 'channel' the air better, but I'm not sure if that is a moot point.
Any way, that is my two cents worth. I might find some more change around it I give it more thought.
Sorry for the cryptic images, but MS Paint didn't seem to have an obvious 'Dashed' line to show hidden lines.
PS. I'm not trying to take over your design (I get accused of that), but you started me thinking about it so I thought I'd throw out some ideas. Please feel free to throw them out yourself if they are not useful.