Similar to the Honl Speed Strap and Thomas Schwenger's lighting kit, this uses double-sided velcro to attach it to the camera. Two strips of non-slip mat, placed on either side of the flash head, stop it from moving around.
The velcro comes from a pack of straps ($8 for 8) made by Tesa. Each strap is long enough to go almost twice around the SB-600 flash head.
To make it truly speedy I'd need to glue the mat to the velcro. At the moment it can be fiddly to tighten up the velcro and keep the strips in place at the same time.
See also a post at DIY Photography Lighting.
UPDATE: I now use a different method for making the speedstraps. The end result is that they're not so fiddly to get on.