Some great ideas here, what a talented group! I really like the idea by Girlfriend of the self-adhesive bags; they certainly seem inexpensive enough and are in line with what I currently do. When I complete a project, I use a ’snack size’ Zip-loc bag to store the left-over matching spool of thread and bobbin.
I also just found some wooden bobbin (tray) holders, filled with bobbins at a church yard sale (25 cents a set) that I had never seen before. They are single wooded racks that remind me of Scrabble letter trays except they lay flat and have magnetic trips across the bottom to hold the bobbins secure. I just got them and unfortunately have not taken a photo of them yet, but I like them and plan on using them to hold my empty bobbins. Perhaps someone else has them that can share a picture. Of course, these are great for metal bobbins but would not work with the new plastic ones.