I'm an active side sleeper, and I often sleep with one knee forward. I find that if I use a quilt that is 8" wider than the measurement from the floor, around my shoulder, and back to the floor, then I don't get woken up by drafts when it's chilly. For a summer quilt, ventilation is easier if the quilt is only 4" wider than the floor to floor girth measurement.
If you are making your own, I'd start wide, with a low taper. Then test it on a weekend trip. If you close the top with widely spaced stitches, you can open up the top, turn it inside out, stitch 1-2" farther in, and trim off the excess.