You could make the lid of a cake board or foam core covered with fondant or gum paste, but if you want to keep it edible you could use a cookie. The covers on these boxes are a cookie covered with a thin layer of cake then covered in fondant:
http://cakecentral.com/gallery/1945387
http://cakecentral.com/gallery/1923349
This one is foam core covered in fondant but if I was to do it again, I'd use a cookie:
http://cakecentral.com/gallery/1923365
If you want the lid on the box, but just slightly open, you would use dowels or skewers to hold it up and a bit of royal icing to cement it in place. Your gum paste or fondant tissue will hide any supports anyway.
Hi, check my photos, I made this cake for the first time two days ago.
I made my cake, standard.
My lid I made from RKT. I put it in the same pan used to make the cake, released, let it cool. Covered that in candy melts, let dry, rubbed with shortening, applied fondant.
Used 6 dowels, at 3 varying heights, to hold the lid at an angle. Tried 4 first but it drooped.
HTH
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