I cut 3 squares off the long side, leaving two pieces - 45" x 70" and 15" x 70"
Before cutting the quilt in two, I sewed two runs the length of the quilt, and then cut in between the runs to minimize down escapees.
Then cut a 3 square by 6 square piece from the 15" wide piece. This piece will be the bottom and end of the footbox. So the footbox is essentially a 3 square x 3 square x 3 square cube, with the one side open.
Attaching the footbox.
And I'm left with a 3 square x 8 square leftover piece.
Finished weight after mod: 14.2 oz.