I was gonna make one myself but I found this in the store while shopping. It was only $20. It amplifies the sound quite a bit. The only thing that I did not like that I corrected was when the iPhone is docked there is only 1 hole that channels the sound. The other side has a little groove that channels the sound not through the channel but out to the open. What I did was took a tiny drill bit & drilled like 6 small holes to get some of the sound channeled through the dock. For the price it is well worth it. I think because it is made of plastic the sound is a bit off, but it is listenable. I am gonna make my own out of a chunk of 4x4. cut it down the middle and use a router to cut the sound groove then glue it back together. I think the sound will be much better with one that is made of wood...
Griffin Technology: AirCurve - Acoustic Amplifier for iPhone