I picked up a couple of cardboard house-move boxes in Lidl or Aldi last year. Dirt cheap and easily big enough for most of my stash.
You have to fold them up yourself but they are very strong with a double layer base.
I also have a pair of large square boxes from Big Yellow Storage that have started to sag at the corners. Not really designed to be stacked but the right size for my *really big kits*!