A round firebox or cylinder is definitely the way to go.
Build your own outside wood furnace.
To start there needs to be 3 pieces of 1 2 steel laid out for the bottom of the furnace.
First you ll choose your product series.
Build your own outdoor boiler here.
Remember that welding such thick materials is quite different from welding 1 8 steel because proper penetration is paramount to your success.
A homemade outdoor wood furnace can be made very simply and run off a small duck fan that is inside the ducking.