For personal use around the house and studio, I started with that idea, but made something quite a bit larger and not quite as polished.
I started with a 12x18" (30x45cm) sheet of 1/8" (3mm) thick white "Funky Foam" craft foam, purchased at a craft store (Hobby Lobby) for just 99 cents.
I held it up against my flash and took pictures of it in a dark room to find out just how much of the foam really got lit up by the flash. I wanted my surface area to be as large as possible for the softest shadows, but I didn't want it to be any larger and bulkier than necessary. Experimentation told me to keep the full 12" width and cut it to 13" long, with the corners closest to the flash tapered at a 45 degree angle (see the title photo).
rubber band clamps" from a woodworking supplier. Three bands from that set fit my flash heads perfectly. For a more professional and more expensive look, you can also use the Honl Speed Strap. You could even glue a few strips of velcro around your flash head and use them to attach all manner of modifiers. I already had these rubber bands, so I didn't count them toward the cost of the light modifier.
|In use with the flash head angled up 45 degrees|
My resulting reflector is quite large -- enough so that I might feel a little conspicuous using it to shoot a professional event. If I were to do that, I would definitely cut a matching sheet of black foam and glue it to the back side of the white reflector. I'd also round off the front corners.
The results are great, especially considering the time and materials that I sunk into the diffuser. You can even point the flash sideways (rotating the foam 90 degrees around the flash head) to put the reflector quite a ways off to one side for some more dramatic portrait lighting -- all on-camera. I used it recently to light many of the interior and engine photos of a truck I'm trying to sell, and the results look far more natural than the bare flash that I initially started using. All in all, it's a great speedlite lighting modifier for photographers on a tight budget.
In the examples below, note how hard the shadows are in the bare flash shots compared with those using the diffuser. As always, click the images for larger versions.
This is especially cool if you're into non-studio macro work. In the lower right photo above, notice how well the immediate foreground is lit. If you were able to sneak up on a tiny bug or flower with that giant white flag over your head, you could emulate a wonderfully soft, overcast sky using just your on-camera flash with this simple diffuser. Gone are the days of your lens casting its own shadow into your scene.
For other dirt cheap DIY light modifiers, read about this soft box, or this formal light tent, or this make shift light tent, or this small parts "light tent."
So what's your preferred method of softening on-camera flash? I'd love to hear any tips for other lighting mods that work really well, regardless of whether they're DIY or commercial products. Please speak up in the comments below.