If you're not worrying about the weight for something like the underbelly you link to, then you could use just about anything. I've jury-rigged up a few using a variety of materials (though mostly with old DIY hammocks). I've had some success with Neat Sheet fabric, with Tyvek, and even with an old cotton bedspread (that last one never saw any camping time, but was handy in the yard for holding my water bottle, etc). For car camping, if I didn't have fabric coming out of my ears at home, I'd probably just go get a bedsheet from the thrift store and then toss it at the end of my trip.