I'm not sure how to get the open look, without the "top of the box" crumbling, do you use cake of rice krispie treats? Any help for all would be appreciated!
Here's a full tutorial on how to make a gift box cake with a lid (they used Styrofoam):
It's heart shaped, but you would use the same techniques on any shape. Hope that helps!
I used RKT on a cake board for my treasure chest. I've also used cake on a board, but probably wouldn't try that for anything bigger than a 9" or 10". use dowels to prop up the lid. I put 3 or 4 along the back edge at the hinge for support (at the height of the lower cake) then 3 long dowels a bit less than halfway back to prop it open. Just put a notch in the bottom of the cake board (not all the way through) for the dowel to rest in, use a bit of hot glue to secure dowels to underside of board/lid.