Yes, I meant to say coupler.
The pot is a Vollrath Optio 27-quart stock pot (1.5mm thick). While not super thick, it is definitely as thick as my first brew kettle, which was a converted Vollrath 38-quart stock pot (what is known as the Classic series today). I had a 3/8" NPT coupler welded into it for fifty dollars. However, that was twenty years ago. and the welder who did the work has since retired.
I would purchase a commercially-made stock pot-based kettle. However, no one makes a nice 6 or 6.5 gallon kettle, and I want to build a 3-gallon brew house with an appropriately-sized kettle. Thirty-two-plus quart kettles are not optimal for sub-5-gallon batches.