Yes, the box sits on top of your top tube but since your top tube slants down the bottom of the box has to somehow conform to this slope. So,

1) the corner of the box that is closest to your stem has to be squished a bit so all the straps can be snugges up, or
2) you could add some velcro to lengthen the bottom strap closest to your saddle and that corner of the box would be unsupported, or
3) you could have the bottom of the box sit parallel with your top tube and lengthen the top-most strap on the side of the box.

Does any of that make sense?

Ack, it would be easier to explain if I could draw a picture. Essentially, it would take some creativity to make it work but it could probably be done.