Hey nice fire box.
Here one we did for a client in West Hollyweird a few years ago.
We build the metal box and encased the thick concrete around it to achive the Dwell / ultra modern look.
At my own house, I have one that's about 2' tall X 10' long that has stone attached to a metal frame with a metal box that has lava rocks in it. It acts as a fence to keep drunks from falling off the edge of my deck (4'-5' drop). Its about 16" thick overall, but it will not pass building code as a fence (need another 12"). I don't have any friends, so they wouldn't fall over, so screw em'. I will try and get some pics tomorrow.