You may have to remove the rear wheel for this to work. Install the bottom bolt first with it near the top of the box. Make sure that while it should be snug, that it is still loose enough for you to slide the box up. Slide the box up to its final position and install the top bolt.

Wow. Thank you so much. I can't believe I lost my temper trying to think so hard about this. 🤦‍♂️