I just use a new (made 27 years ago) British Army 58 Pattern feather Down sleeping bag for vehicle use.



It comes with a non pearmeable water proof base and can be used without a sleeping pad and is rated for -10C. The 58 pattern bag is also reasonably priced for the specification matching top end synthetic bags.

If you want something an synthetic again with a centre zip then the Snugpak SF range is compact (for synthetic) and warm for its weight.