The boiler for a still is made out of sheet copper that is riveted together to form the body of the boiler. How smooth the boiler is depends on your skill with using sheet copper. The bottom of the still is also made out of sheet copper that is riveted to the body of the still. The joints in your boiler can be soldered using silver solder to seal them so that your boiler doesn't leak.
The size of a boiler depends upon how large your still is going to be. The usual size of the still boiler is one that will accommodate 55 gallons, about four gallons of finished product. This is large enough to yield a fair amount of alcohol when it is being operated. This size still is about the size of a 55 gallon oil drum, in fact you can take the dimensions off of one of these drums and transfer it to the size of your boiler.
Once you have finished the bottom half of your boiler you are now ready to build the top. The simplest way is to make a big cone with a four inch hole in the center. This has to be attached to the bottom of the boiler in such a way that you can remove it from the boiler. This is by having mating lips on both parts so they can be pasted together. There also should be an oval hole in the cover for pouring in fresh distiller's beer. This is covered with a copper cover that is also pasted on in such a way that the cover will blow off if too much pressure builds up in the boiler while you are distilling a batch.
The last part of the boiler that you have too make is the elbow. This fits over the hole in the cover, and in use is pasted onto the cover. It also has a bend that brings the elbow a few degrees from parallel to the boiler pointing downwards. This is mde like a long funnel from sheet copper so that its small end will fit onto the worm where the alcohol condenses. The worm also passes through the waterjacket of the still array.
In practice a 55 gallon slug of distiller's beer should produce about 4 gallons of alcohol. When the first distillate comes out of the worm it is charged with several dangerous organic chemicals that are part of the fermentation process and should be discarded. A normal run of alcohol comes out of the still looking cloudy and slightly tea colored. You test for good alcohol by splashing a few drops onto an open flame. If it burns it is worth saving. When it quits burning the alcoholic content is too low. The remaining gallon of distillate should be saved, and run through the still with the next batch.
You can do this five times, then you have to tear the whole still array down for a thorough cleaning. Remember in the distilling business cleanliness is next to godliness.