Ummmm. Is this to be a glider?

The model needs a stout hook (well anchored) just ahead of the center-of-gravity. No idea how to figure it for a deltas, but I used to use a bent bolt in a section of aluminum "C" rail that can be locked in position once the "sweet spot" is located. Too far forward and it doesn't climb as high, too far back and it stalls and the bungee pulls out (before it drags the model for a cartwheeling worm burner, you hope). The hook should be slighty "open" so the bungee ring will easily slide out at apogee.

The bungee itself is a minimum of 100 ft of surgical tubing and one or two hundred feet of nylon "cod line". A large spike at the tubing end and a stout parachute with a ring sewn to the center on the other. We used to double spike the ground end because the thought of that spike coming back in your teeth is a bit concerning. Stretch it back to double the length of the bungee and use a long screwdriver or another spike to hold the tension. Then, when you're ready, hook the ring to the model and ler her rip. First flights should be at only a 30 or 40% stretch on the tubing. Some gliders like a tad of up elevator, some do best with a tad of down. Depends where the hook is positioned.

I REALLY recommend a dummy camera mock-up in the nose of the same weight for the initial test launches. And always hand launch to trim a glider before using a quick-up.