I've used flannel backed tablecloth for lawn chair cushions and for baby changing table mat. Neither were quilted.
I've also used thermal backed drapery fabric for a long cloak. Also not quilted. Very warm and keeps the wind and drizzle off. If it gets really soaked in a downpour it does feel wet inside.
Think you would be best to follow the above suggestion to just put a tarp or plastic tablecloth down under your quilt.