I made one out of one of these:
http://catalog.belkin.com/IWCatProdu...duct_Id=257270
(Belkin tune base for iPod NANO),
and the Belkin clear acrylic shell:
http://catalog.belkin.com/IWCatProdu...duct_Id=374513
I got both of them real cheap at a Ham Radio show (I think I paid about $5 each for them), and I acually did most of the cutting and filing with just a leatherman tool. A little plastic model glue, and I have something that looks much like the bigger Tunebase.
It works, and holds the phone perfectly, but it is a little finiky when you first plug the phone in. Appearently it is specifically trying to talk to an iPod Nano. After jumping in and out of the iPod App a few times, it is fine, and the FM xmtr works. Due to the fact that a Nano has a much smaller battery than the iPhone, it seems to charge very slowly if at all. I need to try it some more, but if I find that it is not at least keeping up with usage, I will have to trash it.