I use a #2 washtub! And a square one at that!!

Seriously, they can be a PITA, as most "fly" boxes are so shallow that they 'crush' the collars if left stored in a closed box for very long (Yeah, I know about steaming them, or running very hot water over them to restore the collar, but who has access to such on the water?). I use the deeper, moveable partioned boxes that the bait casters use for their lures. I can adjust the size of the compartments to fit the bugs.

These boxes are available at a lot of fabric and craft shops; thus, you do not have to go to a tackle shop to find them. I bought a couple of 'fishing gear bags' at Bass Pro a few years back that came with 5 such plastic boxes in them, have two large zippered pouches (one on each end) for reels and extra spools, a meshed pocket for tippet materials, etc., on the outside of each end pouch, and a large zippered pocket on the back side for snacks, or whatever. They have solved a lot of my earlier problems. (No financial interest!)