There's a kit at Thru Hiker that provides materials for a top quilt but those can be easily modified to underquilt duty with end draw cords, corner loops and no sewn footbox. Look at the dimensions and features of a Jacks R Better quilt and you can't go wrong with the underquilt.

Don't like working with down? Order a Ray Way quilt kit for the materials and copy the JRB with it.

Or there's the Red River Gorge. Or any of the other projects at hammockforums.net.

Seriously, hammockforums is a better place to start. Click on Helpful Hammock Articles, then do it yourself gear. There you are.