Hi there, if the lid needs to be closed, then hope this helps.
http://www.semihomemade.com/entertaining/01_tiffany.htm
I make my lids from gumpaste so that they are stiff.. I just use the bottom of my cake tin as my mould.. Make sure that you have greased it well so that it does not stick! And make sure it is completley dry before trying to lift it off.. (will take a few days). HTH 
If it is a closed lid I just add another layer of rolled fondant (slightly thicker) over the "box" and trim it with a knife.
It it needs to be open I cover a piece of foam that is the same size and shape as the cake with gumpaste and then fondant.
Closed Lid- http://www.cakecentral.com/modules.php?name=gallery&file=displayimage&pid=1520614
Open Lid- http://www.cakecentral.com/modules.php?name=gallery&file=displayimage&pid=1522095
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