I just stumbled upon this child seat:
http://www.djgusa.com/press/pressre/totengo.html
I'm currently using a Cosco Alpha Omega in my car (with the seat backs removed) but it requires me to keep the passenger seat way forward which isn't too comfortable. This one seems to make a lot more sense - and it's $30. It appears to be little more than a padded board with a 5 point harness on it that you attach via a lap or shoulder belt. It works for kids 25-40 pounds.
Has anyone tried one of these?
-- I just called the company that makes them and was told Wal-mart carries them for $19.95. I'll post pictures once I've had a chance to pick 2 up and install them.