We have this one:
http://www.homedepot.com/prel80/HDUS....jsp?pn=169134
It is washable, but it looks like it would be hard to put back on. It would be quite easy to line with a blanket or even sew a cover for. It can either sit high or you can use the top half and set it right on the seat. I like that the front is mesh (plastic) so that I can SEE loki and he can see out.
Loki is 9-10 pounds - I can send you a pic of him sitting in it if you want to see how big it is. He curls up in a ball and sleep in it or sits and looks out the window.