I make my regular cakes, decorate and stack them. For the hatbox lid, I make mine from styrofoam. I roll out GP, flip it over, paint it with water, then put the stryofoam on top, and cut around it. Flip it over, smooth it and let it dry. After that I take fondant, cover the top and sides of the lid, flip it over and cut off the excess. Makes a nice seam between the GP and fondant. If you dont want to use the Styrofoam, you can always prop the like open with a wooden dowel.
I prop mine open with a Styrofoam wedge because I usually use that to secure my flowers. (Like a floral oasis). You have to do your flower arrangement first, and keep trying on the lid, to make sure your not going to smash the flowers. I push a floral wire into the bottom of the lid, then push the lid down on the Styrofoam and into the cake. I secure the back of the lid with either a dab of royal icing or white chocolate. Hope this makes sense. Let me know if you have any questions.
I've posted a picture of the babyshower hatbox cake just in case you haven't seen it.