Probably the least expensive way to make up an air warmer unit is to buy a cheap 25 watt (no bigger) soldering iron and clamp it
firmly to a heatsink. Mount the heatsink to the intake fan as shown in my dwgs & photos in the first post so air is drawn through the fins and into the box.
Heatsinks are pretty expensive. The one I am using came out of my junkbox, but probably is worth about $40-50. You can make one easily and cheaply out of aluminum sheet (1/8" thick is good). Cut some sheets of aluminum to a convenient size, bend them into progressively smaller "U" shapes and bolt them together.
Use an incandescent light dimmer to control temp. A standard light dimmer should be able to handle at least 100W, so it will handle the 25W soldering iron element (also a resistive load, just like an incandescent light bulb) with no trouble.
Follow the wiring diagram I included in the first post in this thread. Substitute the heating element from a soldering iron in place of the two power resistors.