Can’t comment on the legalities with the TTB, that is just research with them.
I have also given this a lot of thought and the jury is still out on it with me. I currently own and operate a business that does metal work (not sheet metal or pressure vessels) so I have a bit of knowledge to start with, but at the same time lack a lot of what would be needed to pull it off. If you are thinking this will be a cheaper way to acquire a still, unless you have the practical experience with sheet metals and making pressure vessels this would not be a cheaper way to go. Sure a new commercial still properly set up starts in the $40-50K range but just start adding up the cost of sheet copper. There is a reason meth heads out there are stripping wires from anything not tied down, copper is off the chart expensive these days. Now the metal fabrication I do now, I know the learning curve also comes with a scrap rate and that means $$. One mistake with expensive materials can send you well past the cost of a commercial built, supported and warranted product.
If you do make something, keep in mind you have to use a food grade filler rod for your TIG welding. Also if you make anything that is to take a pressure at all, it will have to be certified as will the welder that put it together. (Like a steam jacket or steam heating coils inside the primary pot)
I can see making hogs heads and general support equipment (racks, drains, drier) but the still its self, just not sure it’s worth it.
Mark