The tube I used was 321 grade as I had a few lengths kicking around but I would think 316 would be good, the tube is 11mm diameter and about 0.7mm wall thickness. I made a 5” mandrel that I put in my lathe and then turned the chuck pulling the tube around the mandrel. You could do the same by mounting a big piece of tube vertical in a large space and walk around it forming the coil.
I have now developed it a bit further and use a whirlpool paddle mounted inside one of the coils driven by a drill to agitate the water.