I find having a body lengths worth of room on one side of the cot (or whatever) is key for getting in and out of your bed.
My setup is fairly simple. Roll-a-cot on one side, Thermarest and sleeping bag on that. Most of my gear and cooler fit under the cot. The rest goes in the front seats...
Mine seems to stay put just fine sitting on top of the dash in the driver's side corner. Maybe the remaining half of the velcro strips from when I had it mounted to the windshield of my previous car help.
