Flexwatt is a USA prouduct so people here use either heat cable (parasene soil warming cable) or heat mats/strips whichever the individual chooses.
So either is fine to use, I think cable works out cheaper but I don't use it because I like strips myself and I'm one of these people where if something can go wrong it will so I could virtually guarantee if I had cable it'd go kaput and make all my racks cold lol.
Like I say heat mats/strips are fine but soil warming cable is another option too.
Pretty much step by step guide on this thread http://www.reptileforums.co.uk/forums/habitat/276765-rub-snake-rack.html
You have to make a groove for the cable, line the grooves with reflective tape to push the heat in the right direction, put cable in on a stat and your done.
Sometimes more than one pass of the cable is needed depending on the size of rub and general temps you achieve.