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.