One of the drawbacks of a hmmock is that it has to be completely covered to prevent rain from filling it up like a bathtub. Any covering has to be longer than the hung hammock.
You could likely do it with GI Shelter halves, although you'd probably need four of them. One shelter half
is, at its longest dimension (point to point), 12'-9",
which means the footprint is considerably shorter. One completed sheter (two halves) would have to be hung rather high in order to cover a 9' hammock, and would not get the job done for an 11' hammock. You could use four halves and overlap them in the middle to get the length you need, but the resulting load would be rather heavy. A completed shelter (two halves) weighs 6 pounds
---without the poles and pegs. This means that to completely cover your hammock, you're carrying 12 pounds worth of tarp.
Using the WW2 style shelter tent that has one open end would reduce the wieght somewhat, but they've become collectible as all get out these days and you'd end up spending as much as you would on a decent cottage vendor tarp that is lighter and purpose designed to do the job.