[Fastera]

Anyway, in my opinion, the best method with smartphones is the sealed box method. What do you suggest to substitute the item? Perhaps old charging cables for smartphones or what else?

An old smartphone? It can work if you are upgrading from an older model to a newer one of the same brand and category. Say you upgrade from iPhone X to iPhone 11 pro. The phone will easily fit in the box and weight difference is negligible (12 grams or something similar).
I am sure there are similar possibilities with other brands as well.