This is pretty neat, seeing as how store-bought drivers seem to run about $20!
Basic idea: strip the mechanics off of an old servo, and solder in two different resistors, so the circuit thinks the alignment is incorrect and supplies power to the circuit. Attach to a spare receiver port.
How To Build A Servo LED Driver: "One way to get into night flying is to fill a plane with LEDs. You can do that by connecting them directly to the motor battery, or to a dedicated second pack, but neither approach gives you any control. Here's how to reuse an old servo to drive your LEDs, with either on/off or variable brightness from the transmitter."