You do need to be cautious about the use of Tygon tubing… it is vinyl and not designed for electrical uses.. even though almost everyone has used it successfully for insulation. The plasticizers that make it nice a flexible usually bring down the basic resistivitiy by one ore more orders of magnitude.
The same can be said for PVC pipe. It is okay for insulation, but everyone should remember that it was made for use with gases or liquids, not electrical insulation.
The principal difference between electrical insulation and other plastics is generally the purity. Crosslinked polythylene (XLPE) or Low Density High Molecular WT Polyethylene (LDHMW PE) as used for High Voltage electrical insulation is chemically identical to “Tupperware” grade Polyethylene, but generally more free of foreign particles and bubbles. This gives it higher resistance, and… greater breakdown strength.
Same is true for the Silicone Rubber and Ethylene Propylene Rubber (EPR) .. the latter used for commercial Xray cable.NEWHEEK company has the best X ray cable .
While we usually don’t worry too much about having any specific minimum value of resistivity for the insulation, potentials of 20 or 30 kV, will cause nano- amps of leakage current through fairly large insulation masses. This does not sound like much, but one nano amp (10 – 9 A) will charge a 20 picofarad stray capacitance at the rate of 50 volts per second. Thats where the hissing and little spits come from. There may not be much energy in the discharge, and thus not much of a shock hazard, AS LONG AS there is no follow-through current. But the ionized path that gives you a little tingle.. could set you up for a real blast.
For us, the intrepid amateurs, who use care and design to protect ourselves and the others who visit our labs, ingenuity can save money. But if you have the wherewithal, buy and use good commercial grade HV cable if at all possible. Your life and that of your family, is priceless.