Wow, what great ideas, everyone.
I have an old Bernina 1530 and one day I finally sprung for one of their expensive boxes to hold the feet in it. Boy, was I disappointed when I ran out of space and also found out that some of the feet really didn't fit into this special box. I was kicking myself for wasting the money.
Now, I have a Janome 6600 and I've been wracking my brain trying to figure out where to put the extra feet (of which some are over-sized) to keep them readily accessable. Now I know.