Public Service Announcement:
Do not use duct tape to attach anything to your flash. The adhesive won't come off, even after an hour of use, and ruin the aesthetics of your flash.
As an aside, turns out duck tape is just an early (military) name for duct tape. Gaffer's tape is basically duc(t/k) tape with a matte surface to avoid reflections and a less sticky adhesive which won't leave any residue. So use that for your flashes (I still need to get my hands on some).
Some duct tape info (and alternate names) can be found at the Duct Tape Guys (crazy site, but worth a look).