Becnal has a good idea with the bivy bag, though wouldn't it also trap moisture in?
Those survival sheets he is talking about also make a good barrier.
Though they are bulky, the old style closed cell foam sleeping pads placed inside can force out the sides of the hammock for a less confined feel. Since a pad can slip around, I had tried the Big Agnes bag/pad combo, as the Big Agnes system doesn't have any insulation on the bottom, just a sleeve for the pad, and that keeps it more in one place underneath you.