I have. But I'm not sure my design is the best. There are several decisions to be made.
One is what type of insulation you would like: down or synthetic. I've never made a down quilt but there is a nice design up at http://thru-hiker.com/projects/down_underquilt.php
Thru-hiker also sells everything you would need to make the quilt.
The second design question is shape. You will notice that the shape of the quilt at the link above is sort of egg-shaped, to match the bottom of the hammock when you are lying in it. However many people think a rectangular quilt does just as well. You will notice that the underquilts at Jack R Better (probably the #1 seller of underquilts) are rectangular. The suspension bunches the ends up when you hang it, much like your hammock bunches up, so the two match. A similar choice faces you with synthetic insulation. If you look at the Kickassquilts at http://arrowheadequipment.webs.com/kickassquilts.htm
you will notice that they sell both an egg-shaped (the Patomac) and a rectangular (the New River) synthetic underquilt. The suspension on New River quilt is simply a cord through a channel on the ends.
For my hammock I made a rectangular 48x72 quilt using 5 osy Climashield. Couldn't be easer. Just sewed the insulation to the two layers on three sides and then turned it inside out and sewed the end closed. For suspension I put grosgrain loops and then zigzagged a line between the loops and my suspension lines. I posted a picture at hammockforums. Not sure of the link below will work or not. But the underquilt works great.http://www.hammockforums.net/forum/picture.php?albumid=156&pictureid=1412
The last question is length. Mine is a full length quilt. Some people like a 3/4 length to save weight.