Hi there,
They are not as hard as you think.
You make the lids out of foam and cover them in the same colour fondant. Make the cake for the 'box' and cover it either by wrapping the fondant around the sides and make the fondant a little higher than the cake to give it some depth or cover it as usual and then add a lip later.
I have attached a couple of pictures, not quite sure what sort of box you are wanting to make but I hope these help
http://www.cakecentral.com/cake-photo_1425029.html
http://www.cakecentral.com/cake-photo_1420592.html
http://www.cakecentral.com/cake-photo_1419286.html
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