the fan you want moves the air slowly. the air has to slowly travel through the greenhouse box cans or ducting to heat up.I like the concept...
I do have power in my coop but a heater would be a safety issue. However, something like this is doable without the safety concern.
Did you watch the video of the guy who did this for his man cave, by chance? (https://www.treehugger.com/how-build-diy-solar-air-heater-old-soda-cans-4856770)
I would do the "flow" in reverse from what he did... I'd use the fan to pull air from the ceiling, pipe it through the cans/ducts to warm it and circulate it back to the room near the floor.
I'd only be using this in the winter so grabbing warmer air from the ceiling to be heated through the system would be more efficient because you'd already be a tad bit warmer to start. And pumping it out the bottom to slowly rise through the room so it disperses better...
Of course that's provided your fan is strong enough to suck in the air and also push it through the system to the bottom pipe.