I use one of these
Clickit Sport - Sleepypod® | The safest pet company | Dog Carrier | Cat Carrier | Dog Harness . Not a barrier, but it does keep my dogs in the back seat. It's the only brand I can find that passes crash testing. Before the ClickIt Sport harness, I had tried Clickit Utility harnesses with less success. The dogs weren't comfortable in the Utility harness, and one even cut his leg on a strap. When the Sport harness came out, I was able to exchange the Clickit Utility for the ClickIt Sport, and our problems were completely solved. The dogs are comfortable, can change positions easily, and are safely restrained in the back seat. I like that the ClickIt makes the ride safer for passengers by keeping the dogs from flying forward during a crash, and I definitely like that it keeps them safer by keeping them on the seat and reducing impact.