Forum member Madmike recently joined the forum with the announcement that he was just finishing up making a solar hot air panel
The panel is 45 inches wide by 90 inches tall and 4 inches deep. It sports 16 rows of pop / soda cans that are 15 cans long for a total of 240 cans. The frame is made of 1x4s and a 1/2" plywood backing. The inside of it is lined with 3/4" foil faced foam. The glazing is a sheet of polycarbonate (lexan).
To punch holes in the cans he used a 1-1/2″ hole saw and a drill press on the bottom of the cans. For the top of the cans he simply took a tin snips and bent the fins at an angle to help create some turbulence which will promote heat transfer to the air.
The panel is still getting tweaked now with a larger and higher flow fan to get the outlet temperature down a bit and increase the efficiency (less heat lost to the environment). To control the fan he is simply using a snap disc thermal switch that turns on at 110F and turns off at 90F. To prevent backflow through the collector at night he is using a dryer damper.
For more details on the project see Madmike’s forum thread.