Maybe, I could use something harder to penetrate. For example, if I paid something too expensive and want to avoid custom payment, I could put it in concrete column, small concrete blockhead, highly enriched with hematite elements to block the x ray penetration, make that blockhead as some sort of artistic obelisk or grave monument that is too precious to break. I know that concrete gets darkened color when mixed with such additives, but it could be painted to avoid suspicion. Also, it can be thinner than regular concrete and act as one monolith structure to hide whatever is inside. Since I've seen that these, 'special' concretes are having far more density than normal concrete. If they can protect nuclear reactor from dispersing its radiation, then it could be applied here too. If you see a blockhead which looks like any other concrete block, seeing nothing inside through x ray scanner, then there's no suspicion.