I have seen a full size mummy case done for a school art project.  It was very impressive and could, with a bit of adaptation (i.e. shelves) serve as a cupboard.  With something like that you have to be careful with the weighting, making sure it is not top heavy.  If you build the whole thing on a lightweight card structure, you could incorporate a bag of sand in the base section to give it solidity.
To think of internal strength, try putting a few toilet roll cores side by side with a board on top.  Stand on them (carefully) to see how strong they are.  You can adapt engineering principles with e.g. triangular section cardboard tubes.  The ideas are endless.
Good luck with whatever you choose.
DavidO