The walls of the sarcophagus are quite thick.  You can simulate this and make it quite strong by cutting lots of strips of cardboard about 1" wide and laying them as spacers between the inner and outer surfaces.  I think I would probably lay the outer surface flat on the floor, spot glue the spacers all over it, put dabs of spirit glue or PVA on the top edge of the spacers and then lay the inner surface on top of that with something to weigh it down flat.  The five sections can then be joined together.

If you're following Catperson's (Sue's) idea about relief moulding in pulp, be careful about moisture getting into your base boards - it could soften them completely.  To overcome this you could lay a thin film of plastic over the base, model the PM and let it dry thoroughly, then remove the plastic and stick the relief onto the framework.   I think the idea of brushing PVA and then sprinkling sand is very good;  it will give a nice 'stone' like texture.
DavidO