I've used the above seat belt clips too. They work great, but if you want to run doorless, I'd probably go another route. When I have my dogs with me (one is Rottweiler) I keep the back seats folded down. I used paracord to braid a few clips that connect their harnesses to the trail rail system. Making your own, you can make it exactly the length you need/want to allow your dog some room to move, yet keeping it short enoungh that they can't get out the doors. When doing this ALWAYS use a harness. If you crash you don't want you best friend's momentum being stopped by a thin belt (aka a collar) around their neck. What's worse than crashing you brand new Jeep, is crashing your brand new Jeep and your dog being killed because it got choked out or it's neck broken.
Here's a photo of the harnesses we use. You can get the patches to say a lot of different things, such as nervous, pet me, do not pet, etc. On his nose is a gentle leader (a head collar, not a muzzle) which is what his lead hooks into. That hangs disconnected when riding, and the harness, which has a D-ring under the handle is clipped to the Jeep.