Jason wrote, "I like the idea, foldability and weight of polycro but has anybody been in a heavy rain under a tarp with it? I feel like if water started running down under your tarp you couldn't prop up the polycro to keep it off your bag?"
It doesn't take much of an edge. You can rake up a little duff with your foot to form a berm under the polycro if it looks like there might be run-off under your tarp--- better than digging and making a mess of the campsite and it can be scattered when you leave. More often, I use a larger size than my tarp perimeter often just roll the edge under, forming a small berm a few inches inside of the tarp edge.
Frost King has a large window insulation kit that is 62"x210", so you can get a couple large ground cloths from one. Make sure it is for outdoor use (Home Depot Model # V95QP Internet # 202262329)
Tyvek is tough, but heavier, bulkier and noisy when new. I like it for a tent footprint to protect the floor.