Doubt you will find an off the shelf RCA to Cigarette converter.
You could get an RCA audio cable, cut off one end, and wire it up to a cigarette plug from Jaycar or Dick Smith.
I can make one for you and send it to you BUT I'm not happy that you are going to run it flat out! I'm sure they are designed to receive controlled 12V. You risk overheating your secondary and will reduce the life of the heater?
A little controller can be knocked up for about $30-40. I could make it up when I can get the time. PM me if you wish.
Not sure about insulating material. Does the heater wrap around the stalk well inside the shadow of the Secondary? Then you can go with a synthetic wool or something - a bit of thickness will be OK. But you should think about maybe a dark felt "flocking" type material - several layers of that - Something white and/or shiny won't be the best idea.