The secondary structure of a tesla coil should be made of a substance of low loss. Materials such as polycarbonate (Lexan) or acrylic (Plexiglass) are appropriate.  The coil form should be as thin as feasible, but must have enough strength to support the winding procedure and to sustain the Toroid terminal. A Tesla coil secondary has its own particular electrical makeup determined in part by the length of secondary. Like a tuning fork, it wants to vibrate at a particular frequency. The secondary is inductively energized by the primary coil.

The secondary for this 900 machine is a five inch form wound with AWG24 magnet wire on a 5 inch plexy glass form with a sealed top and bottom. The five-inch secondary is 30 inches long and has about 29.5 inches of windings. There are 48 windings per inch. 1416 windings total, 1852 feet of wire were used to wrap this coil.  The running frequency is 132.9 KHz. The last picture to the left is comparing the 900 watt 5 inch secondary with the 1800 watt 6 inch secondary. Tab on pictures to enlarge.