My daughter and I have made two DS cakes and one briefcase cake with the lids standing open. I used straight gumpaste for the lids. After coloring the gumpaste I rolled it out and then using the same pan the cake was baked in. I laid the gumpaste on the bottom of the pan and molded it around the edges then cut the edges even. Then on which ever will be the "bottom" of the lid I made an extension that would be the same as the height of the cake, then turn under another inch. Once the lid is dry the part that is turned under will be underneath the cake with the extension going up the back side of the cake and adhered with melted chocolate.
I will post pictures of the cakes in my profile for you to look at.
Someone else may have an easier way of doing it but for me this works pretty well.