Strongboxes refer to sturdy containers designed to house valuable items securely. There are numerous synonyms for strongboxes that you can use to replace the word in your writing seamlessly. Some options worth considering include safes, vaults, lockboxes, receptacles, and robust containers. Other synonyms may include trunks, chests, caskets, and crates. The right synonym to use depends on the context of your writing and the specific type of container relevant to the discussion. Regardless of the word chosen, the synonym should fit the overall tone of your text and provide an accurate description of the strong, secure containers used to store valuables.