Is it possible to hand-make a GSM-interference shield?

I started using my iPhone in my car instead of my dash-mounted iPod running through a cassette adapter. I love everything except the occasional GSM noise. If I cup my hand around the base of the phone where the Belkin charger/audio adapter plugs in, the noise goes away.

Is it possible to make some kind of homemade shield or guard I can attach to the bottom of this setup to block that GSM noise?