I bought a foam cup cell phone holder at a truck stop for $5. It sits in a cup hole in my car, and can easily hold both my iPod and cell phone. It has two slots - one big, one small, and four holes (for pens, Blistek, etc).

If you took one of those, and gently widened the slot with a razor or X-Acto knife, it would work fine. Plus, since the power adapter socket is on the bottom of the unit, a plug can snake out the bottom hole, to your lighter socket. It was built with a hole in it, by design, for charging cell phones. Similarly, a cassette tape adapter could plug into it, and also snake out the bottom hole.

Just a thought.