I think that is one you can technically add a firebox too. Just check the right side of the unit and see if it has the football shaped cut out area. If so you could add a box. However it might run realy hot since the main chamber is so small.
If you do have the football knock out be sure to read up on the do's and don'ts of knocking it out. Having a 2x4 to support the area you are hitting with a hammer is a must! Also if you can get to a dremel with a cut off wheel it works well to pre-cut the small welded areas a bit. Other than that a phillips screwdriver and a deadblow hammer are all you need.