Everything can be contained in something like a plastic lunch box. If needed, a frame could be constructed inside this to support the equipment.
We can make a harness to wrap round the lunch box, and we can attach that to the balloon and parachute lines. If it's strong enough, we can just attach the lines to holes in the box.
It consists of four 1/8“ ply sheets, two of which are half size in length. On the side that is four sheets thick, there is a hole to attach the payload.
On the other side, a bolt is passed through both sides to hinge a servo horn in the middle. I have used a few washers to space it out. In a hole about half way along the servo horn (it can be trimmed if you wish!) I have passed a piano wire pin through both sides of the ply to lock this servo horn in place. The line for the balloon can then be attached to the servo horn, and the pin can be pulled out by a servo to release it. The servo is just glued to the top and secured with a cable tie.
The parachute line will be permanently attached to the payload.
The balloon line will be a loop. One end of the loop is permanently attached to the payload. This will then pass through a loop attached to the balloon (so they are loose), and the other end will be connected to the balloon release (so it can be released!). The balloon release is then permanently attached to the payload.
The parachute can be supported by something held up on the permamently attached side of the balloon line loop by a knot. The other side of the balloon line loop can pass through the parachute next to this weight.
If the balloon bursts, the weight will pull the balloon line clear of the parachute, and the parachute will activate. If we cut down (release the balloon release), the payload will pull this end of the loop out of the loop on the balloon, and the weight will pull the line clear of the parachute, and the parachute will activate.