I just completed my first hot air collector. I used black polyester felt as an absorber behind a 6.5ft x 32 in glass door. the air flows through the felt. It is a convection system at this point, but a pv fan may be added later. I got the felt idea from " build it solar" It was a joy to work with compared to metal, and looks better too. Lets hope it works. I will let you know.