I don't work in fondant and do primarily wedding cakes. I am fascinated by the talents of those who make treasure chests and hatbox cakes in which the lids is slightly open.
For my own curiosity, how to you keep the lid standing partially open like that? It's an amazing effect that I'm sure I'd never master.
Mine isn't amazing by any stretch of the imagination....but I actually used foam as the lid, wrapped in foil and iced with buttercream then used dowel rods to get to open at an angle.
Some on here are true professionals and I'm sure a lot more creative, but it seemed to work for me! Hope this helps. 
(Nah..........!)
I did cake for a hatbox top one time but I didn't think it looked as good...recently I did both a treasure chest and a Tiffany box, and had the (hollow fondant) lids open. I tried dowels but my fondant wasn't rock hard and the dowel began to push through, so I just laid it on at a "stable angle" I guess, and secured with royal icing, and refrigerated it and it stayed just fine...
Mocakes your treasure chest cake is awesome!!!
DianeLM also has an amazing treasure chest cake posted and the lid is all cake. The talent on CC amazes me!!!!
Hi. I used Rice Krispy Treats covered in fondant for my treasure chest lid (in my photos). Then I laid the lid on a piece of cardboard covered in wax paper (or freezer paper- can't remember), slightly smaller than the lid. I put in some dowels that were a little taller than the cake, where you couldn't see them kind of in the middle of the cake/lid, a little bit toward the front. Was sturdy and held it open just enough. 
Hope that helps.
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