The reason you get cold is you compress you bag insulation at certain points. When you are on the ground, the ground and/or sleeping pad serve as insulation but in a hammock there is only cold air below. So, you must get some insulation that doesn't compress below those areas where your bag compresses (on me it is my hip and shoulder).
The quilt above is the best solution IMO. I have a Hennessy and I sewed some silnylon on the underside and filled it with down. There are commercial solutions as well.