Silnylon works well. I made a Ray-Way style tarp from it, and the only disadvantage I've found is that it tends to collect a fine coating of dust, I assume due to static electricity.

A long time ago my then-girlfriend made me a 10'x10' tarp of 1.9oz. PU coated nylon. It worked well, but heavy by today's standards (about 2 lbs.).

3- or 4-mil polyethylene, usually only available in black or clear, but occasionally in light gray, also works. The corners can be secured with sheet bends, but you need Grip Clips or similar for other pullouts:

https://www.shelter-systems.com/grip-clips-tarp-and-fabric-fasteners/

Note, by the way, that Grip Clips won't work on silnylon since it's so slippery. I've read that you can make it work by putting a thin piece of rubber (cut from a balloon) between the clip and the fabric.
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Always remember that you are absolutely unique, just like everybody else. -Margaret Mead