It's not necessary anyway, at least in the US. From the National Cremation FAQ:  Is a casket required? No. For sanitary reasons, ease of placement and dignity, many crematories require that the deceased be cremated in a combustible, leak proof, rigid, covered container. This does not need to be a casket as such. What is required is an enclosed, rigid, container made of wood or other combustible material to allow for the dignified handling of human remains. The type of casket or container selected is really a personal decision. Caskets and containers are available in a wide variety of materials ranging from simple cardboard containers to beautifully handcrafted oak, maple or mahogany caskets.   They specifically mention that cardboard is fine.