This approach seems to give me a better result:
It should be the natural way - without duplicate and reverse the spirals as in first try. Also, in that situation I was not able to weld or even use boolean operation in order to get just one object.
The bridge operation over that thin extrusion produced an interesting result if I apply Smooth Preview:
It should be the natural way - without duplicate and reverse the spirals as in first try. Also, in that situation I was not able to weld or even use boolean operation in order to get just one object.
The bridge operation over that thin extrusion produced an interesting result if I apply Smooth Preview: