I personally use PVC pipe for the construction of the barrell. But you can use almost anything, I have even heard of people using steel and copper. If you want to take a gamble with electricution using metal barrell's well thats you, just atleast put a few layers of HV tape or sealent seperating the barrell and the coil. Ok now in your end formers or spacers drill 2 holes, now insert 1 end of the wire into the front spacer. Then just tightly roll the coil, try not to make gaps these just create problems and bumps down the line. Once your done you will want to put a few layers of insulation just to be safe over the top of the coil. Since the coil has a tendency to snap or move, I use lashing wire or high stregnth metal band fasteners to hold the coil down. This is a real problem getting when delaing with thousands of watts.