For more than 40 years, I've made a series of potstills from aluminum canning pressure cookers (3-5 gallons), for me and for friends. I have never worn one out, although I lost one
. I've even had one that had spent wash left in it for over a year, with no serious pitting. I did have to rubber-mallet the lid off, though (it was one of the gasketless taper-fit jobs). I still have one for smaller still runs, and I can swap still heads easily between that and my newer 62-liter stainless still.
Although it might not work with the flimsy metal of a stockpot, I get a small plate of thicker aluminum (1/4" works fine) welded to the lid. I then drill it and tap it to accept NPT fittings. The last few times I standardized on 2" NPT, because I now have that tap.
I've told a lot of people that stainless is the best still material, right up until you try to machine it or pay someone to weld or machine it. I still recommend the large aluminum pressure cookers, often available for a song at rural garage sales, for a first real still.
That's my story and I'm stickin' with it. (Of course, I've never tried to use a stock pot, which is a whole 'nother thing)