It's totally doable to stand the doubles up yourself and then keep them balanced by holding onto one of the shoulder straps. You get one arm in this way then kinda lean them into your back/shoulder as you shimmy the other arm in. Might take a little practice but we do this all the time at Lobos. Much easier with a buddy holding it up while you get your arms in.
+1. Lots of us GUE types around here use doubles with their pickups or SUVs. Bonus points if you have a flat floor, bumper, or tailgate at the perfect height to rest on while you chew the fat with other divers because you don't want to be walking around with the rig on your back any more than you have to. A lifted truck is probably not a good idea unless you're taller.
Now, I did have a sedan previously with doubles, and what I did was to lay down a thick rubber mat on the trunk opening, which let the bottom of the doubles rest against the bottom edge and the sides against the weatherstripping on the top edge of the trunk. The doubles will lie somewhat vertically, maybe 15 degrees off vertical depending on the shape of your trunk opening, and the rubber mat keeps pretty secure.
It was a pretty reasonable solution and didn't hurt the car if you were careful. It was easier than having to raise them up from laying horizontally, but I eventually gave in and got a 4Runner and it's been a great dive vehicle.