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.

5 seconds sounds like a lot for a free falling drop tank..

Had to drop it from high altitude!

Actually the deployment time is adjustable by shortening the kevlar thread.
I think the knots may have slipped a little over the 5 years or so that this thing has been gathering dust in my garage. I'm pretty sure it was around 2-3 seconds when I was using it.

BTW Matt, I found the best parachute material for this was the plastic from a thin black trash bag, and kevlar thread for the strings. Strings were run all the way across the "canopy" material (crossing at the center/top) and taped in place. Then an appropriate strength of monofilament attached the chute to the back of the tank and that acted as a break-away in case the chute accidentally deployed before the tank was dropped.

geez this thing looks like it belongs next to the Thomas Edison exhibit at the Smithsonian.

I had a long line that tied to the plane so after the tank dropped about 6 feet from the plane it pulled the parachute out. My problem is the chute was packed so tight it did not open no matter how high I got, it was an afterthought and a big enough chute to slow things down was a tight fit in the space i had.

But the overriding problem I had was knowing if the fuel was actually burning out of the drop tank. I used a combination tank fitting and drop fitting, (a metal Luer fitting) and that worked, but it was hard to seal completely when attached to the model. So when you flew if you pulled a bit of air into the mains, you did not know it and ended up dropping the 80 ounce tank with 20-30 ounces of fuel left in it (dropped like a stone) and you were 30 ounces short in the onboard mains. (oops)