In reply to damowilk:
> We have a PHD down duvet, combined with a double sized Exped mat, for luxurious car camping. The problem is the duvet is so light, it doesn't stay put. It needs some way of attached it down to the corners of the mat.
This was just what I was going to say. I've got an ME down quilt for the van and the only issue is when you move, so oes it, it's so light it kind of floats, so if it's cold you can get a chilly breeze under it when you move.
A normal duvet for car camping is fine, just make sure here's enough insulation under you.