Silnylon works well as a windblock, - but the trick is to have some stretch to it to get it to conform a bit to the hammock bottom.

The poncho I'm using is basically a big rectancle of silynlyon, 5x8, with a shockcord (stretchy) sewn in one of the 5 foot lengths on the bottom. this lets me tie the other 5 foot lenght at one end of the hammokc, and the shockcords at the other, giving some stretch to it. I can then stick some stuff in between the poncho and the hammock bottom. Works pretty good, esp with a blue pad or two in the hammock.

My only other suggestion would be remember fleece clothing. puffy insulating layers compress under you, and you get cold. Fleece does not nearly as much, so you stay warmer.

You can ditch out the $$ for a new sleeping bag, but I find for warmth in a hammock, what's under me matters as much or more.