How to connect transformer to outlet for humidifier
If I were in your shoes, I'd use a small box (a handy box with a blank cover is A-OK) placed at the rigid to flex junction and supported from a metal hangar strap attached to the ceiling (vs. mounting boxes to the ductwork, which isn't such a hot idea) to hold the "tap" connection to the furnace power wires, and then run a flex (or a MC cable, your pick) over to another strap-suspended box that mounts the transformer (as 24V HVAC transformers generally mount directly to a box) and a single 120VAC outlet for the humidifier's power cord.