One suggestion,
I'd use the rocket tube for the engine holder. Cut it so it's about 80% as long as the engine, seal it on the front end and slide the rocket into it with a little bit of tape for friction to hold it in, but not secure it semi permanent. I had this happen on a rocket a long time ago. The nose cone had more friction than the motor and the motor got ejected and not the nose cone. ICBM time on that one. Anyway, back to the project, it looks like you bend sheet metal pretty good so I'd make a shield like the one you have but also make a box for the rocket pack (if you use multiple rockets in the pack). When the burn is over the parachute charge will eject the motor(s) automatically.
Just a thought.
Whitt...