Using Duct Tape is a horrible idea. It's not resistant to heat like foil tape. Foil tape is meant to take high temperatures. Duct tape is asking for a huge mess as the adhesive is going to be all gooey and will shift your flexwatt position on the rack.
I use electrical tape to cover all exposed areas on my flexwatt and open wires, then use the foil tape to secure the flex down to the rack. Voila! No 'live' connection between the flex and the foil tape.