I JUST purchased this one on eBay,
Pet Gear Large LookOut Dog Car Safety Booster Seat 25lb - eBay (item 180426800325 end time Dec-30-09 11:59:40 PST) It was on sale so I'd check that out before spending more money... but anyways, I haven't got it yet so I will let you know how I like it. Everyone's reviews here on YT seemed to LOVE it!
I have read online that some people thought the small was too small... even for their tiny dogs. I'd opt to go one size up (it's only $10 more) so she can be more comfy and lay down more.