Since I have no certainty as to the ideal proportions or dimensions, my approach is purely empirical and the use of waste scrap serves me to decide in this area of design!
In the picture, the vertical tube (riser) is attached to the fireplace; The latter consists of 50 angles welded in a square to form a square tube. As can be seen, the entrance of the gases is strongly offset, so as to initiate a vortex without using a restriction of the section, as is generally the case (which seems to me nonsense). It remains to make a cut in the vertical tube to make the junction of these two parts. So far, the fireplace parts are simply pointed at the arc (3) and this will probably be sufficient, as a perfect seal is not required at this level.
Next step, cutting the vertical tube to a height of 100 and about half the circumference: this is more than what is necessary, but I prefer to cut further and detach the part that will ensure the junction Of spiral primer), because it will be easier to form outside the tube ...
(1) It is important to start from recovery, because this is a first realization that will probably require modifications ...
(2) From 6 M long! What to do a lot of tests ...
(3) On rusted scrap (even slightly prepared), it is the best welding process: the mig does not appreciate ...
You do not find? 
And the instructions seem to be increasingly strict).
