Nice looking Pit!! You got more patience than me. To answer the Warming box ? It's do able you will need to vent excess heat VIA an additional stack and opening with a Damper to the main Chamber so if you want it hotter you close down on the damper.
Here is a good visual from a build a friend did he forgot more than I'll ever know.http://www.texasbbqforum.com/viewtopic.php?f=4&t=18210