I use one similar to what wizard posted, I love it and it works great. Tiggy is great in the car, and the most he does when he decides to wander is stand with front feet on the middle console, he does know "get in the back" and stays there most of the time. The only time he comes all the way up (even when we stop and/or get out) is when invited/told he can. Well I take that back, for some reason when we pull into the garage Tiggy always tries (and has succeeded) to sit in my husbands lap while he parks; it is the only time he has ever does this.
But I really use the seat belt almost all the time, because even though he's good about staying in the back, he doesn't lay down right away and will look out the windows or out the back which means he puts his front feet up. With that if I were to have to make a quick stop he'd most likely end up in the front or hurting himself more because he's not secured as well. For Reggie, I usually only use his if we are going to be on the highway. Maybe I should use it more, but Reggie gets in and lays down right away, if he does sit up to look out he's leaned back against the seat and sitting, he very rarely stands up.