Even easier is 3 speed blower motor resister block from any '60 -'70s GM car. Use one of the three to get the speed you want, mount it in the air stream to keep it cool just as GM did. You can find new ones for around $14 from any auto parts store. Splice into the your current low speed wire, and add the resistor. I used one on a Vintage Air under seat heater, I mounted it right in front of the blower wheel intake opening, nothing fancy, just one screw to hold it in the air stream. I used the biggest resistor of the three to really slow down the motor. If you are real creative, you could add an additional switch to make a 5 speed fan.
