Hiya, You can do a shoe box cake lid several ways. The easiest is to keep it as part of the cake and just put a flap on fondant over the top as a lid. ….like in this photo of a cake I did. You could also have the lid separately – either made of RKT(Rice Krispie Treats like I did for the cake in the second photo. You could also cover a real box lid from a Dollar store with fondant and place that on the cake in an open box position…and lastly you could cover a piece of Styrofoam with fondant for the lid. It’s best to keep all elements of a cake edible – but that’s a personal choice – you just have to inform your customer about it.
Mel SugarMama <3