As it happens, I'm about to design and make a combined topper/cozy for internal AND external use. My raison d'être is simply to do with the cost of these items on the after-market caravan, when suitable material is available for under £50. There's little science in improving warmth and dryness at this level so it's worth a try. Lots of people use these toppers in different ways, and you have to choose whether to have internal, external, thermal, or God forbid, all three! I want ONE solution that can be deployed in any way you choose, be easy to make, easy to store and cheap.
A lot of people say GoreTex, breathable, and the like, but these fabrics only really work (if at all) when they are close to a sweaty body, whereby some saturated airborne moisture can pass through the fabric pores under pressure. I don't think this is going to add any benefit to our purpose here. We want waterproof, windproof, a "double glazing" effect, and continued ventilation. A cheap waterproof ripstop fabric is ideal for this.