For simplicity it's hard to beat an 8x10 silnylon tarp from Campmor. I sewed some tie points in along the centerline so I could hang it from an overhead line rather than drape it over. That way, it's easy to pull either or both sides down in response to weather. You can do the same thing with a ball+loop clamp or even just bundling up duff and lassoing a line around the clump. That size was perfect for three when I brought two kids along.
A flat tarp doesn't scale well for one person, however. A pyramid seems to make more sense in terms of weather protection per ounce of material. I got lazy and bought a Deschutes from Six Moon Designs but it shouldn't be hard to make something similar from Silnylon or whatever.
I definitely wouldn't trust Polycro for overhead protection. I used some as a groundsheet once and it got a huge rip in it when I dragged it a few feet with my stuff on it. This is NBD for a ground sheet but would ruin your night if it got poked by a branch during a rainstorm.
For dew and condensation protection clothing-style Tyvek (not the thicker house wrap) makes a nice combination ground cloth and blanket. You can pull it over your face without suffocating or making condensation inside. I wouldn't trust it in real rain but I've been fine in a light mist followed by heavy morning dew when a companion cowboy camper woke up soaked.