Key Idea:  Run a 12 volt fan on 6 volts for low noise.

Details:  Get a fan about 3 inches square and tie or bolt it to the back of the final cage.  I used lacing tape, but ty-raps are probably easier.  The lamp behind the frequency scale has one terminal connected directly to the 12.6 VAC filament supply.  Put a single rectifier diode on that terminal.  Put the anode on the terminal and the cathode on the red fan wire.  Ground the black fan wire.  Leave enough slack in the fan wires to get the final cage off without trouble.  The fan runs almost too quiet to hear.