I really like the Sunpak filter kit for my 555 flash. It comes with two diffusers, an 85 filter, and several colored filters (red, blue, green, etc.). One diffuser is for 20mm, and cuts two stops. The other cuts one stop, and I believe it is for 28mm. They also make a version of the kit with smaller filters for the 383 flash. If you get the filter holder for either of these kits, it would probably be very easy to jury rig that holder to a variety of flashes, rather than taping the filters on.