Firebox design

Check out this post:  http://www.smokingmeatforums.com/t/256059/having-a-smoker-built-in-mexico

I think the thing with having the firebox under your smoking chamber is how you control the heat.  You will probably need to have a water pan between them.  There are many bullet style smokers that do it this way.  I think the Big Green Egg uses some kind of heat deflector plate.