You could also put them in a cardboard/parcel box, and then bungie the box to rack. Those cardboard boxes usually found at the back of shopping centres which used to contain packets of (e.g.) snacks.

If you bungie those to the rack, then maybe the pressure from the bungees go over the boxes, not over the styrofoam. Since the boxes are pretty much free/widely available, you can discard them when dirty, or just inconvenient. They also come in various sizes—more so than tupperwares—so one of them would suit.

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