Here is another great one from Martin Kimeldorf, converting a LumiQuest popup diffuser into a softbox thingy. This will work great to add some direct flash fill in. Now, I am not saying that you should run and buy one of those just to mod it, you can start with silk and some black Bristol and get the same place.
I liked the LumiQuest Diffuser for Pop Up flashes except that the diffuser panel hung rather limply on my camera and thus produced an unreliable surface shape. Also, I wondered about the light escaping the sides. Then I thought, I could add sides and create a miniature soft box, especially if the inside had a silvered lining.