As mentioned in my previous blog entry, shown below is my DIY diffuser. Material used is a plastic binding cover (bought in National Bookstore). This plastic has 2 surface finish: coarse & smooth. For the inner side of the diffuser, I used the coarse side so as to have consistent 'scattering' of light.
Again, I would like to emphasize that you'll only use a diffuser when you have (preferably white) walls or ceilings around you. Otherwise, you'll look stupid for trying to bounce/diffuse light off the clouds (now what if it's a cloudless day?). Seriously, you're just wasting power.
If you really need to use flash outdoors, you can do the following:
1) Bounce light off a reflector.
2) Use as fill light (during bright daylight)
3) Use your flash off-camera via extension cord or an RF trigger system.
Remember, photography is basically lights & shadows.