Joz, I did something almost exactly like what you described for one of my neat sheet underquilts. I got some flat transparent elastic band material from JoAnn that weighed less than an ounce for something like a 30-foot length. I stretched that moderately while I was sewing it along the outisde of the quilt (no hem to speak of on the neat sheet). As long as the corners of the quilt are snug to the hammock, this works really well to eliminate gaps. It's light, cheap, and easy. When I'm out of the hammock and the quilt is relaxed, however, the bunched edges make it look like a giant blue diaper.