For emergencies I'd go with a $4 roll of 2 mm painters drop cloth cut into whatever size I liked. Stick a pebble in the corner, gather, tie off with guy lines.

Or a poncho tarp - double duty. There are heavy cheap ones with grommets or light silnylon ones with grosgrain tie out loops.

Or a silnylon tarp from one of the many cottage gear makers out there.

Or, what I take for SAR, a contractor bag - can be a bivy, a liner for a bark n branch shelter, a ground cloth, a sit pad, a water carrier, or cut it open for a nice large-ish tarp. Seven bucks a roll for contractor bags. Big enough to tie out with a trekking pole for a one person tent in a pinch. Probably about the same dimensions as those tube tent things they still sell, but heavier plastic and more uses.