I don't understand the point of having the pipe in there at all.
Why not just have a black box with no pipe? Intake at one corner, outlet at the diagonally opposite corner. If you feel you must have a serpentine airflow, add a few horizontal wooden baffles starting from alternate sides and going most of the way toward the opposite side, stopping about the baffle-to-baffle spacing from the side to give the air a good passage to the next horizontal path.
Want better light catching? Fanfold some metal, partially unfold it, paint it black, and mount it on the back fo the container. Light will take multiple bounces going down the fold before it can get back out, improving absorption. If your point of the pipe was to keep the hot air separated from the outer glass for insulation, mount your collector nearer the front and do your air harvesting on the backside of it. Or just use two layers of glass.
If you must put the air in a serpentine pipe, at least have the runs horizontal, rather than vertical, so you don't have to repeatedly pump the hot air downward. Convection should be your friend, not your enemy. Heck: Unless you want to force the air down to the floor level you can let convection pump it for you.