A round firebox or cylinder is definitely the way to go.
Build your own outside wood furnace.
Build your own outdoor boiler here.
It is really just a steel box inside of another steel box.
You can certainly build a clean burning outside wood burning furnace given access to the right materials and sufficient welding skills.
Remember that welding such thick materials is quite different from welding 1 8 steel because proper penetration is paramount to your success.
The furnace builder allows you to customize the product the size and the features of your outdoor wood boiler so that you don t pay for anything you don t need and so you get all the features you want for the most affordable possible cost.