>>1649339>Make a box with a black-painted peice of trash roofing in it that is covered with glass or plastic. Allow air flow into one end and out the other end. Lean it up slightly so the sun can heat the air inside and send it into your tunnel/greenhouse. Put a lightweight plastic flap over the inner hole on the greenhouse side so it acts as a check valve when night comes and you don't lose the heat through it.Like this, only I sorta messed up on how deep the roofing corrugation is. While corrugated roofing is optimal for surface area, but flat roofing metal also works. Air flow can be adjusted with a sliding vent on the bottom end. This sort of design is also used to heat homes where a window can't be placed, though air from inside circulates out to the solar heating unit instead of outside air coming in.
Whatever you do, don't use that small-holed soda can version that's floating around online. It has terrible air flow and is a massive amount of work making it an extremely poor choice of design. Fast adjustable air flow is key.
Keep in mind, if you have more barrels and more plastic, you can simple extend/widen the tunnel/greenhouse using those for a similar effect with the bonus of extra thermal mass. It'll be cheaper and easier.
