I use an army poncho liner like Just Bill mentioned. I bought it in "like New" condition off Ebay for $25. In summer, it is my primary top quilt. In winter, I drape it over my ridgeline outside the bug net to create a wind barrier. It has tie outs sewn around the outside edges for easy attachment. Lowers temps in the hammock by around 10 degrees.
Another summer option would be a Costco down comforter. They are only available during winter and cost around $35-40. I bought by one off Hammock Forums that a member turned into a sewn foot box quilt for $50 delivered. It is extremely light, comfortable and perfect for summer temps. They should be available at Costco in a few months.
Carl