Only thing I know to do is get a universal mount for a phone this size and make sure the USB connector for charging isn't blocked by it. Otherwise, no one has made a mount specifically for the Eris yet.
Having just installed the leaked 2.1 ROM, I am newly in the market for a car mount myself. I, too, noticed a few at Best Buy that looked serviceable. I think I saw a universal at Target that didn't look too huge. I'll report back if I try one out.
