Obviously fits great on Numa Compact series since it's tailor-made for it. While it is a great functioning and great looking keyboard backpack, I am 1.64m tall and I had the problem where the bottom of the bag would hit the back of my legs when walking. Initially I just had to walk a bit hunched to angle the keyboard away from my legs.
Then I realized that the backpack straps were not in the vertical center of the keyboard back, and wearing the backpack upside down actually places the keyboard a bit higher and now I don't have any problems walking with the backpack on. I wish this was the default design, but my workaround works, and I would just recommend to buy attachable shoulder pads to put on the "bare" side of the strap.