Metal containers are also available.
I visited the local Goodwill store and bought a couple of small , used steel gift boxes for either 50 cents or 99 cents each.
Some Goodwill stores even have plastic models donated by folks, though variety is understandably unpredictable. Some were old ESCI 1/35 diorama accessories; most are half assembled car kits.
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I will admit I actually bought a small plastic case from Wal Mart prior to buying the metal boxes.
Ya oughta see some of the stuff being sold in those Goodwill stores.
Even Tesla car owners are visiting the stores in search of treasures.