Wow, those are just like my grandfather's old canvas tarp ... except in modern fabrics.
Hmm ...
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I have one of these 9.5 x 10 tarps. It's great, tie outs everywhere, 16 oz. and under a hundred bucks. And they have sales. I think I only paid $75 for mine on sale. You can close the ends like doors and still have enough room for a 10' hammock. Or you can set it up in a Bakers Hut. It's a very good tarp.
http://www.bearpawwd.com/tents_tarps/tarps.html
I say the best tarp that provides the best coverage for its weight is a hex cut tarp.
"Best" depends on what you want it to do.
If it's "cover your hammock from straight down rain at the least weight and bulk possible", an asymmetric tarp is your friend.
If it's "keep you warm in cold, blustery weather", a wide tarp with doors (also known as a "winter" tarp for obvious reasons) is great.
If it's "cover your hammock in serious downpours with wind during warmer seasons while still providing an area to cook/hang out under", an hex cut tarp is good.
If it's "cover your hammock in inclement weather and still be usable if/when you choose to sleep on the ground instead of comfortably in an hammock", a rectangular or square tarp is great.
Hope it helps!