There's a sticky thread in the Cake decorating forum called "My hatbox cake directions." Here's the link: http://cakecentral.com/cake-decorating-ftopic-215781-0.html She does an awesome job of explaining how a hatbox cake (and lid) are put together.
Also Sugarshack just came out with her best DVD yet -- Boxes and Bows!! (WELL worth the $$)! 
How do You make the lids for the box cakes? I'm guessing a cake board but not sure...
For my gift cake I made the lid using an inverted cake pan. Simply dust the bottom of the pan with corn starch. Roll out your gumpaste and cover the bottom of the pan. Use a knife to trim the edges of the gumpaste. Build your bow directly on top of the lid. Let this all dry. I let the one in my gallery dry in less than a 12 hour period. I would sugest letting it dry longer. Once it's all dry remove it from the bottom of the pan and slip a cake board underneath. I then used chopsticks to support the lid on the cake.
Here is the link to the gift box cake in my album that I made using my method. If you need more help feel free to PM me.
http://cakecentral.com/modules.php?name=gallery&file=displayimage&pid=43231
Sharon Zambito of Sugared Productions (and known here on CC as Sugarshack) has just released a "Boxes and Bows" DVD. If you can swing it, you'd find it an awesome resource.
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