Kate did this:
THe thing to keep in mind is the UQ should be pulled up to the bottom of the hammock with SHOCK CORDS... let the hammock carry your weight, and the shock cord keep it snugged to your backside.
Draw cords on each end shape the rectangle to your hammock. . . don't make those too tight... On heavy sleeping bags, you might have to cut darts (cut out long,skinny triangles, short side on the edge of the quilt) to reduce the amount you have to pull in.
GOOD sleeping bags make passable quilts. Heavy/cheap bags don't conform well, and are really heavy, making poor quilts without a lot of fiddling. light kid-sized sleeping bags work reasonably well.
Just my $0.02