You can buy flashers off ebay quite cheap, but they don't look like they stop the flashing and just remain on after a few seconds. Which will drive people behind you insane I would imagine.
http://www.sentex.ca/~mec1995/circ/motflash.html and looks to do the trick. Shouldn't be more than a fiver of components on some strip board.
I'd want to go a bit further and only enable the flashing at over 20mph or so.
BTW I don't think it is legal in the UK.
EDIT***
Also done this now, with some Maplins Waterproof led strips.
No idea what it looks like in the light of day yet and the wiring isn't that neat, as I did it in the dimly lit garage
EDIT 2: Finished the LED install in the topbox
