I kept the parachute timer/ejector from my last drop tank.
It is fairly simple. Free flight timer mounted on front with a long arm. There is a spring loaded piston inside the carbon fiber tube, the piston is attached to a piece of 5/32" music wire that extends through the brass rod at the front. There is an notch cut in both the brass tube and the piston rod that align when the piston is fully compressed. A piece of smaller music wire is spring loaded to fit into the notches when they align. That way when the piston is compressed it is automatically "cocked", and all that is required to release it is to pull the small music wire out of the notch.
That is where the timer comes in. The timer is attached to the small music wire with a little piece of kevlar thread, when the timer arm is in the up position it releases the piston. The timer arm is held in the down position by the bottom of the airplane when the drop tank is mounted, it is automatically released to start "timing" when the tank separates from the plane.
Operation:
The timer arm is held down. The piston is compressed to the "cocked" position. The folded parachute goes into the carbon fiber cylinder. Then the tank is mounted on the plane.
When the tank is released from the plane the timer counts down about 5 seconds before pulling the trigger and releasing the piston. The piston pushes the folded chute out the back of the tube. In a perfect world then the chute unfolds itself in the airstream and opens, which allows the drop tank to slowly float to earth.