I have a great hotshoe flash (AF-540) which I try to use whenever possible, but there are nevertheless times when I must go with the pop up flash...and since I mainly shoot manual lenses, the flash output (on the K20D) cannot be attenuated below full power, which most often results in some blown out images and very harsh shadows.
I did try the film can trick but aside from a way to dial down the output, i didn't find this to soften any shadows.
Today I experimented with something I found in the pantry, a crystal light container, which i shortened and nested half inside the other half, sandwiching a piece of Kleenex between layers of this white polypropelene shell. I taped a small rectangle of foil just in front of where the pop up bulb fires to block the hard beam, and then applied foil at both ends and in the rear to keep the light aiming forward. The whole thing took about 20 minutes and i did it while sitting in the yard, so it is not very refined, but i was surprised to see it actually works well to soften the shadows.
Here is the device on my camera. Yeah, not the purtiest thing you ever saw:
It slides on easily with a slot and then the endcap snaps on to hold the whole thing together. If it gets windy, i'd probably use a piece of tape to secure it to the camera so it doesn't wobble.
Check out this before and after, particularly the darkest shadow in the center of the frame:
Pop Up Flash (naked)
With DIY diffuser
i'll experiment a little more to try to position it with the widest side aiming forward; when i cut it, it wanted to lay down as in the pic, but so far am pleased with this DIY contraption.