Lumiquest has a pretty nice solution for diffusing on (or off) camera strobes by dressing them with a small and light weight softbox.
Reader Julian Schüngel came up with a similar device that can be constructed from one A4 sheet of paper (and kicks the crap out of my similar design that needs 6 sheets). Oh yea, it has build in barn doors too.
Now, while the side panels still emit light this can be easily corrected by turning them black, which Jullian indeed added for version #2.
Click the image below to see the full tut complete with sizes and detailed instructions on Julian photostream
The second version tut is in the following picture. It uses some extra paper and one more square cut of the front, but it eliminates the light spill.