I used 2 packages of light boning (like that used in shirt collars rather than used in functional corsets) and sewed it in a spiral "spring" to give the case a bit of structure. It was a bit annoying to do because I had to sew it in by hand by shoving a fish food can in the tube to keep from sewing the walls of the case together. It was also the most expensive part of the case and I don't know how much it really helps getting the bo in and out. If I make one again, I will probably leave this out.
I used the handle and strap from a discarded piece of luggage. Having both a shoulder strap and a handle is very useful.
I kept the top rigid by sewing in a pipe cap for a PVC pipe and the bottom by pushing a stiff foam can cozy to the bottom to keep the bo from wearing a hole in the bottom.
I end up carrying multiple sticks (1 bo, 2 han bo, 1 jo) in the case and it makes it much, much more convenient to get to class.