Last year I bought a cheap kingsize 16.5 Tog duvet for £20 or less. I have it over my tank and hose reels and the small oil filled rad between the reels and RO. Seems to work and even keeps the warm tank still warm the next morning. Everybody who looks in the side door ask if I have a bed in the back of my van. When I reply no they come back with why do you have a duvet in the van so I have now jumped to the chase and rely the quilt is to keep my water tank warm.