What about a weather balloon with the perfect amount of hydrogen in it?
What about using a real human instead of a human sized bag?
What about parking a crane outside and hanging a steel cable down through the middle of the building?
😉
Diagonal beams from which corners? Top corners?
Why not from the ceiling? Aren't the top corners, technically the ceiling too? If it is too weak, why not put in a joist spanning the ceiling or just reinforce the existing ceiling support?
Otherwise, it seems like you'd want something similar to a punching bag stand except beefier and deeper footprint to support the extra weight, for example:
I don't understand why using the same methods anyone else uses, won't work.