Spark plug wire is ok so long as it is not of the ¡®resistive¡¯ type, (I¡¯m dating myself with that!). For all my projects above 1KV, I use boiler igniter wire. This stuff is good to 20KV and has a silicon insulation. I¡¯ve use other that was sort-of ¡®double insulated¡¯, (main insulation with an outer jacket). But that was extremely stiff and I often though if too tight a bend were made, the insulation would break! I¡¯d hit the local auto parts jobber to see what they have available or maybe an electrical store. Also on an afterthought, some test-lead cable is also rated to 6KV. A few options for you.
