What with one thing after another (big jobs, odd jobs, truck problems, etc.), it took us a while to get the barn's cistern done. However, we made it. It is now finished, so our fish and water plants can be warmer and our water isn't lost to evaporation.
We made the frame out of 1/2" PVC, reinforced with rebar. It arches north to south and then connects in straight (ish) bits down to the east and west. We tied the frame down to the cables attached to the posts.
We then stretched greenhouse plastic over the whole thing. We attached this plastic to the frame using chunks of 3/4" PVC. These chunks had a third of its circle cut out and were sanded. They fit very snugly and hold the plastic very well.