Sorry I have not designed such a bath, however have you gone to the site
?
This person seems to be knowledgeable in this matter.
Like all engineers, given enough time and money I could apply the rules of science and design such a building but it may very well be easier if you can find someone with experience designing exactly what you want.
The first question you need to think about is what fuel is available, gas, electric, or wood, also what type of wood?, Different species have different fuel values.
Also where is the bath building located, what is the design temperature difference from inside to outside.
How well insulated is the proposed building?
How tight are the doors and windows, will there be a lot of infiltration?