If I'm transporting tanks to/from the dive site/boat they are in the trunk between the rebreather and gear box. No need for a way to secure them as they can't roll around sandwiched between things. If I'm taking tanks to get fills, they are either in the backseat on the floor (one or two tanks), in the trunk with the pool noodles others have posted (2+ tanks) or in a milk crate for the case of rebreather bottles. For longer road trips, I'll stand them up against the back seat and use a ratchet strap to secure them upright. This way there is more room for the rest of the gear.