I held the speakers in place with two blocks of expanded polystyrene foam.
Wash and dry the soup cans thoroughly. Press the cans onto the foam to give you a rough size to cut. Cut circles of foam, but trim ~ half an inch off two sides, you'll need this to get the foam into the cans (see later). Press the speakers onto the foam to give you a rough size to cut. Cut enclosures for the speakers, in this case the rectangular holes grip the magnet assemblies. Taper a round edge to accommodate the speaker-cone, and ensure there's room for wire to pass through the foam block.