@TEDthebed Well you won't know if a hot water bottle works for you unless you try. You can just use the bottle to initially warm the bag and take it out when you get in. Or eject it later if you feel it is not doing much. You can try it at home to get comfortable with it.
It is also easy to try a blanket over the top of your sleeping bag. For car camping you can just take one or two from home and see what it takes.