The box (see below) is equipped with two insert-tab-and-velcro locks and closed quite securely.
The box itself weighs next to nothing but nevertheless guards your miniatures quite well against unwelcome dents and scratches. Multiple boxes would be easily stackable. Safe and Sound also sells bags to pack multiple boxes.
Certainly a nice surprise was the price. Being a stingy Dutchman even I could find no objection in 8 Euros per box. I am quite impressed with this product and thought it good to share it with you.
The (Polish) company has boxes available in several sizes on their site, most of which is in English. Price range from 7 to 15 euros, S&H not included.