Responsa zu Dewarim 32:43

May we not say, therefore, that while indeed it is not the custom to use a metal casket, yet these soldiers who gave up their life for a righteous cause be deemed righteous and therefore do not necessarily need direct contact with the earth; that therefore the casked be not disturbed or pierced but they be buried in the earth as received. Moreover, all these bodies had previously been buried overseas in wooden coffins and indeed some without coffins. Thus, the purpose of burial in contact with earth (namely, atonement, Deuteronomy 32:43) has been amply fulfilled.