I made one last year from a gossamer gear spinsheet ground cloth (www.gossamergear.com/cgi-bin/gossamergear/spinn_sheet_ground_cloth.html). While I consider it still in "testing," so far so good. In use, imagine a slingback using hiking poles as a frame. Sorry - this is clearly a case where a picture would be worth a thousand words...I don't have one at the moment.
All I did was fold one end over about 5 inches and sewed in a slightly tapered pocket with seams that are reinforced with judiciously place grosgrain ribbon. To use it, I shorten my 3 section hiking poles to their minimum, lash them at the mid-point (a clove hitch around both seems sufficient), form an X, place the handles into the pocket with the groundcloth spread out, prop the working ends into the ground so that the poles angle back, and ease myself into place. Body weight holds the show together. I've found that I can make "getting in" easier by placing my pack on it to hold it in place - and then either prop my feet up, or shove the pack out of the way. Net weight gain is nearly nothing, giving that the groundcloth does double duty.
Quite comfy - though you need to add padding if you want to get your fanny off the ground. In practice, I don't find it necessary. The seams have held up better than I expected - I suspect that most of the stress is directed straight into the corners of that pocket - which is where I focused my reinforcing. Time will tell.